Microbiology
microbilogy practice exams
Test your knowledge with our Microbiology Practice Exam featuring easy, medium, and hard questions. Each item comes with a clear explanation to help you understand the correct answer and strengthen your exam skills. Perfect for ASCP MLS and MLT exam prep!
- Which type of bacteria is most commonly associated with urinary tract infections (UTIs)?
A. Staphylococcus epidermidis
B. Escherichia coli
C. Pseudomonas aeruginosa
D. Proteus vulgaris
✅ Answer: B. Escherichia coli
Explanation:
E. coli is the most common cause of UTIs, responsible for 70–95% of cases. It is a Gram-negative rod and part of the normal intestinal flora, but can infect the urinary tract due to proximity and specific virulence factors like fimbriae.
- Which of the following is a Gram-positive, spore-forming rod that causes anthrax?
A. Bacillus anthracis
B. Clostridium perfringens
C. Listeria monocytogenes
D. Corynebacterium diphtheriae
✅ Answer: A. Bacillus anthracis
Explanation:
Bacillus anthracis is a Gram-positive, aerobic, spore-forming rod known for its role in anthrax. Spores are highly resistant and can survive in soil for decades. Unlike Clostridium, Bacillus is aerobic.
3. A microbiologist accidentally breaks a tube containing a positive TB culture in a BSL-2 lab. What is the most appropriate immediate response?
A. Wipe the spill with ethanol
B. Continue working and report later
C. Evacuate and notify the biosafety officer immediately
D. Dispose of the sample and resume work
Answer: C. Evacuate and notify the biosafety officer immediately
Explanation:
Mycobacterium tuberculosis is an aerosol-transmissible, BSL-3 pathogen. Accidental exposure in a lower containment lab is an emergency. Immediate evacuation, containment, and incident reporting to biosafety authorities are required to assess risk and initiate post-exposure protocols.
- Which of the following organisms is oxidase-positive and often associated with cystic fibrosis infections?
A. Klebsiella pneumoniae
B. Staphylococcus aureus
C. Pseudomonas aeruginosa
D. Escherichia coli
✅ Answer: C. Pseudomonas aeruginosa
Explanation:
Pseudomonas aeruginosa is a Gram-negative, oxidase-positive rod. It’s known for producing a blue-green pigment (pyocyanin) and a grape-like odor, and is a common cause of lung infections in cystic fibrosis patients due to its biofilm-forming capability.
.
- Which microorganism is the primary cause of bacterial pharyngitis?
A. Streptococcus agalactiae
B. Neisseria meningitidis
C. Streptococcus pyogenes
D. Haemophilus influenzae
✅ Answer: C. Streptococcus pyogenes
Explanation:
Streptococcus pyogenes (Group A Streptococcus) is the most common cause of bacterial pharyngitis (strep throat). It’s a Gram-positive cocci in chains and is beta-hemolytic on blood agar.
- What is the purpose of a catalase test in microbiology?
A. To identify lactose fermentation
B. To detect oxygen production
C. To differentiate Staphylococcus from Streptococcus
D. To determine antibiotic sensitivity
✅ Answer: C. To differentiate Staphylococcus from Streptococcus
Explanation:
The catalase test detects the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen. Staphylococcus spp. are catalase-positive, while Streptococcus spp. are catalase-negative, making this a key differential test.
- Which of the following bacteria causes diphtheria?
A. Corynebacterium diphtheriae
B. Clostridium botulinum
C. Listeria monocytogenes
D. Haemophilus influenzae
✅ Answer: A. Corynebacterium diphtheriae
Explanation:
Corynebacterium diphtheriae causes diphtheria, a potentially life-threatening infection that produces a gray pseudomembrane in the throat. It is Gram-positive, non-spore-forming, and club-shaped.
8. Which of the following organisms is commonly detected using acid-fast staining?
A. Salmonella typhi
B. Neisseria gonorrhoeae
C. Mycobacterium tuberculosis
D. Treponema pallidum
✅ Answer: C. Mycobacterium tuberculosis
Explanation:
Mycobacterium tuberculosis has a waxy cell wall rich in mycolic acid, making it resistant to Gram stain. It’s best visualized using acid-fast staining (Ziehl-Neelsen stain).
- Which organism is known for producing a “swarming” motility pattern on blood agar?
A. Escherichia coli
B. Proteus mirabilis
C. Serratia marcescens
D. Enterobacter cloacae
✅ Answer: B. Proteus mirabilis
Explanation:
Proteus mirabilis, a Gram-negative bacillus, exhibits a distinctive swarming motility on blood agar due to its flagella. It also produces urease, contributing to urinary tract stones.
- What is the primary virulence factor of Streptococcus pneumoniae?
A. Pili
B. Endotoxin
C. Capsule
D. Exotoxin A
.
✅ Answer: C. Capsule
Explanation:
The capsule of Streptococcus pneumoniae is its major virulence factor, helping it evade phagocytosis. It’s alpha-hemolytic, bile-soluble, and optochin-sensitive. Capsule detection is important in the Quellung reaction.
- Which test differentiates Neisseria gonorhea from Neisseria meningitidis?
A. Catalase
B. Oxidase
C. Sugar fermentation (carbohydrate utilization)
D. Gram stain
✅ Answer: C. Sugar fermentation (carbohydrate utilization)
Explanation:
Neisseria gonorrhoeae ferments glucose only, while N. meningitidis ferments glucose and maltose. Sugar utilization tests are key for differentiating between these two Gram-negative diplococci.
- Which anaerobic, spore-forming organism is associated with antibiotic-associated diarrhea and pseudomembranous colitis?
A. Clostridium perfringens
B. Clostridium tetani
C. Clostridium botulinum
D. Clostridium difficile
✅ Answer: D. Clostridium difficile
Explanation:
Clostridium difficile is an anaerobic, Gram-positive, spore-forming bacillus. It produces toxins A and B, which damage the colon lining, leading to pseudomembranous colitis, often after antibiotic use. Diagnosis involves toxin detection in stool or PCR.
- Which of the following organisms causes whooping cough?
A. Haemophilus influenzae
B. Mycoplasma pneumoniae
C. Bordetella pertussis
D. Legionella pneumophila
✅ Answer: C. Bordetella pertussis
Explanation:
Bordetella pertussis is a Gram-negative coccobacillus that causes pertussis (whooping cough). It produces pertussis toxin, which paralyzes cilia and contributes to the characteristic “whooping” cough. It is diagnosed with PCR or culture on Bordet-Gengou or Regan-Lowe media.
- Which fungal organism is most commonly associated with diaper rash and oral thrush?
A. Cryptococcus neoformans
B. Candida albicans
C. Aspergillus fumigatus
D. Histoplasma capsulatum
✅ Answer: B. Candida albicans
Explanation:
Candida albicans is a yeast that causes superficial infections such as oral thrush, vaginal candidiasis, and diaper rash. It forms germ tubes in serum at 37°C, which is a rapid test for identification.
- Which bacterium is often responsible for gastroenteritis from undercooked poultry and produces a “seagull wing” shape on Gram stain?
A. Campylobacter jejuni
B. Shigella sonnei
C. Vibrio cholerae
D. Salmonella typhimurium
✅ Answer: A. Campylobacter jejuni
Explanation:
Campylobacter jejuni is a curved, Gram-negative rod that appears as “seagull wings” on Gram stain. It’s a leading cause of bacterial diarrhea, often associated with contaminated poultry and Guillain-Barré syndrome as a rare complication.
- Which test is most appropriate for confirming the diagnosis of syphilis after a positive screening test?
A. VDRL
B. RPR
C. TP-PA or FTA-ABS
D. ELISA for HIV
✅ Answer: C. TP-PA or FTA-ABS
Explanation:
Treponemal tests like TP-PA (T. pallidum particle agglutination) and FTA-ABS (fluorescent treponemal antibody-absorption) are used to confirm syphilis after a positive non-treponemal test (VDRL or RPR). These tests are more specific for Treponema pallidum.
- Which anaerobic organism is associated with sulfur granules in draining sinus tracts and is often misdiagnosed as a neoplasm?
A. Actinomyces israelii
B. Nocardia asteroides
C. Clostridium perfringens
D. Bacteroides fragilis
✅ Answer: A. Actinomyces israelii
Explanation:
Actinomyces israelii is a Gram-positive, filamentous, anaerobic bacteria that forms sulfur granules (yellow, pus-like material) in draining abscesses, especially in the cervicofacial area. It can mimic malignancy due to its mass-like presentation.
- Which of the following organisms is a dimorphic fungus endemic to the Ohio and Mississippi River Valleys?
A. Blastomyces dermatitidis
B. Coccidioides immitis
C. Histoplasma capsulatum
D. Cryptococcus neoformans
✅ Answer: C. Histoplasma capsulatum
Explanation:
Histoplasma capsulatum is a dimorphic fungus found in bird and bat droppings, particularly in the Ohio and Mississippi River Valleys. It grows as mold in the environment and as yeast in the body. It’s often seen inside macrophages on histology.
- Which parasitic organism causes hydatid disease and forms large cysts in organs such as the liver?
A. Echinococcus granulosus
B. Taenia solium
C. Schistosoma mansoni
D. Entamoeba histolytica
✅ Answer: A. Echinococcus granulosus
Explanation:
Echinococcus granulosus is a cestode (tapeworm) that causes hydatid cysts, most commonly in the liver. The cysts can be large and multilocular. Rupture can cause anaphylaxis. Dogs are the definitive hosts; humans are accidental intermediate hosts.
- A patient presents with pneumonia-like symptoms after cleaning a birdcage. Which organism is the most likely cause?
A. Mycoplasma pneumoniae
B. Chlamydia psittaci
C. Legionella pneumophila
D. Histoplasma capsulatum
✅ Answer: B. Chlamydia psittaci
Explanation:
Chlamydia psittaci causes psittacosis, a zoonotic pneumonia acquired from infected birds (parrots, pigeons, etc.). It is an obligate intracellular organism and is part of the Chlamydiaceae family. It is diagnosed using serology or PCR.
- Which organism is most likely to cause meningitis in a neonate and shows a CAMP-positive reaction?
A. Listeria monocytogenes
B. Streptococcus agalactiae
C. Escherichia coli
D. Neisseria meningitidis
✅ Answer: B. Streptococcus agalactiae
Explanation:
Streptococcus agalactiae (Group B Streptococcus) is a Gram-positive cocci that is beta-hemolytic and CAMP-positive. It’s a leading cause of neonatal meningitis, sepsis, and pneumonia. Pregnant women are screened and treated to prevent transmission.
- Which bacteria produce a characteristic “triple sugar iron (TSI) slant” reaction with a black butt due to hydrogen sulfide (H₂S) production?
A. Shigella sonnei
B. Escherichia coli
C. Salmonella enterica
D. Yersinia enterocolitica
✅ Answer: C. Salmonella enterica
Explanation:
Salmonella enterica produces H₂S, which forms a black precipitate in the butt of a TSI agar slant. This differentiates it from Shigella, which does not produce H₂S. It’s also motile and lactose non-fermenting.
- Which diagnostic method is most effective for identifying Mycoplasma pneumoniae?
A. Gram stain
B. Acid-fast stain
C. Culture on chocolate agar
D. PCR or cold agglutinin test
✅ Answer: D. PCR or cold agglutinin test
Explanation:
Mycoplasma pneumoniae lacks a cell wall, so it doesn’t Gram stain. It’s identified using PCR or cold agglutinin testing (IgM autoantibodies that agglutinate RBCs at cold temperatures). It’s a common cause of atypical pneumonia.
24. Which of the following organisms is urease-positive and plays a role in the development of gastric ulcers and cancer?
A. Campylobacter jejuni
B. Escherichia coli
C. Helicobacter pylori
D. Vibrio cholerae
✅ Answer: C. Helicobacter pylori
Explanation:
Helicobacter pylori is a spiral-shaped, Gram-negative bacterium that is urease-positive. Urease neutralizes stomach acid, allowing the organism to survive. It’s strongly associated with peptic ulcers and gastric adenocarcinoma. Diagnosed via urea breath test or biopsy.
25. A 32-year-old man presents with fever, headache, and a stiff neck. Cerebrospinal fluid (CSF) Gram stain shows Gram-negative diplococci within neutrophils. Which of the following is the most likely causative organism?
A) Haemophilus influenzae
B) Neisseria meningitidis
C) Listeria monocytogenes
D) Cryptococcus neoformans
✅ Answer: B. Neisseria meningitidis
Explanation:
Neisseria meningitidis is a Gram-negative diplococcus that commonly causes bacterial meningitis, especially in young adults. On CSF Gram stain, it typically appears inside neutrophils because it is phagocytosed during infection.
Haemophilus influenzae is also a cause of meningitis but appears as small Gram-negative coccobacilli, not diplococci.
Listeria monocytogenes is a Gram-positive bacillus, not Gram-negative.
Cryptococcus neoformans is a fungus, best seen with India ink stain or cryptococcal antigen test, not Gram stain.
26. Which bacterial species is known for producing a characteristic grape-like odor on culture media?
A. Escherichia coli
B. Staphylococcus aureus
C. Pseudomonas aeruginosa
D. Klebsiella pneumoniae
Answer: C. Pseudomonas aeruginosa
Explanation:
Pseudomonas aeruginosa emits a sweet, grape-like or fruity odour due to the production of aminoacetophenone, particularly when grown on agar like Mueller-Hinton or blood agar. This feature, along with its blue-green pigment (pyocyanin), helps differentiate it from other non-fermenters.
27. In the Gram stain procedure, what is the primary function of iodine?
A. Acts as a decolorizer
B. Fixes the bacteria to the slide
C. Serves as a mordant to fix crystal violet
D. Enhances the contrast of safranin
Answer:
C. Serves as a mordant to fix crystal violet
Explanation:
🔵 Iodine acts as a mordant by binding to crystal violet to form a large complex (crystal violet-iodine), making it harder to wash out from Gram-positive cells. This step is essential for accurate differentiation between Gram-positive and Gram-negative organisms.
28. Which of the following organisms is most commonly associated with “rice water stools” in cholera?
A. Vibrio vulnificus
B. Shigella dysenteriae
C. Vibrio cholerae
D. Enterotoxigenic E. coli
Answer: C. Vibrio cholerae
Explanation:
🔵 Vibrio cholerae produces cholera toxin, which causes rapid secretion of electrolytes and water into the intestinal lumen. This results in profuse, watery diarrhea often described as “rice water stools,” a hallmark symptom of cholera.
29. Which method is most appropriate for testing antimicrobial susceptibility of anaerobes?
A. Disk diffusion
B. Broth microdilution
C. E-test
D. Kirby-Bauer
Answer: C. E-test
Explanation:
🔵 The E-test is widely used for anaerobes because it can determine the Minimum Inhibitory Concentration (MIC) on solid media while accommodating anaerobic conditions. Disk diffusion isn’t reliable for anaerobes due to their slow and variable growth.
30. What is the most likely identification of a Gram-positive, catalase-negative cocci arranged in chains that shows beta-hemolysis on blood agar?
A. Enterococcus faecalis
B. Staphylococcus aureus
C. Streptococcus pyogenes
D. Streptococcus pneumoniae
Answer: C. Streptococcus pyogenes
Explanation:
🔵 Streptococcus pyogenes (Group A Strep) appears as Gram-positive cocci in chains, is catalase-negative, and typically demonstrates beta-hemolysis on blood agar due to its production of streptolysins. It’s also sensitive to bacitracin.
31. A urease-positive, Gram-negative bacillus isolated from a gastric biopsy is most likely:
A. Proteus mirabilis
B. Helicobacter pylori
C. Escherichia coli
D. Klebsiella pneumoniae
Answer: B. Helicobacter pylori
Explanation:
🔵 Helicobacter pylori is a curved, Gram-negative bacillus that produces large amounts of urease to neutralize gastric acid. This trait allows it to survive in the stomach and contributes to pathogenesis in peptic ulcer disease. Urease breath tests are often used to detect H. pylori infections.
32. Which of the following is a dimorphic fungus commonly associated with bird or bat droppings in the Ohio and Mississippi River valleys?
A. Aspergillus fumigatus
B. Histoplasma capsulatum
C. Candida albicans
D. Cryptococcus neoformans
Answer: B. Histoplasma capsulatum
Explanation:
🔵 Histoplasma capsulatum is a dimorphic fungus endemic to certain U.S. regions. In the environment, it exists as a mold, and within host tissue, it appears as small intracellular yeasts—especially within macrophages. It is commonly contracted by inhaling spores from contaminated soil enriched with bird or bat droppings.
33. Which parasite causes autoinfection and may result in hyperinfection in immunocompromised patients?
A. Entamoeba histolytica
B. Strongyloides stercoralis
C. Trichuris trichiura
D. Schistosoma mansoni
Answer: B. Strongyloides stercoralis
Explanation:
🔵 Strongyloides stercoralis can complete its life cycle within the human host through autoinfection. This mechanism allows the parasite to persist for decades and can lead to disseminated strongyloidiasis—especially in immunosuppressed individuals—causing widespread organ damage.
34. Which bacterial toxin inhibits the release of inhibitory neurotransmitters, leading to spastic paralysis?
A. Tetanospasmin
B. Botulinum toxin
C. Diptheria toxin
D. Shiga toxin
Answer: A. Tetanospasmin
Explanation:
🔵 Tetanospasmin, produced by Clostridium tetani, blocks the release of glycine and GABA from inhibitory neurons in the central nervous system. This inhibition leads to sustained muscle contraction and spastic paralysis, as seen in tetanus.
35. A stool sample reveals acid-fast oocysts measuring 4–6 µm. What is the most likely parasite?
A. Cyclospora cayetanensis
B. Cryptosporidium parvum
C. Isospora belli
D. Giardia lamblia
Answer: B. Cryptosporidium parvum
Explanation:
🔵 Cryptosporidium parvum oocysts are acid-fast and measure 4–6 µm in diameter, making them smaller than Cyclospora (8–10 µm). It causes profuse watery diarrhea, particularly in immunocompromised patients, and is transmitted through contaminated water.
36. A Gram-negative diplococcus that ferments glucose but not maltose is most likely:
A. Neisseria meningitidis
B. Moraxella catarrhalis
C. Neisseria gonorrhoeae
D. Acinetobacter baumannii
Answer: C. Neisseria gonorrhoeae
Explanation:
🔵 Neisseria gonorrhoeae is a fastidious Gram-negative diplococcus that ferments glucose but not maltose. In contrast, Neisseria meningitidis ferments both. Identification is confirmed through carbohydrate utilization tests or molecular methods.
37. Which virus is a common cause of severe diarrhea in infants and young children and has a characteristic wheel-like appearance on electron microscopy?
A. Norovirus
B. Adenovirus
C. Rotavirus
D. Astrovirus
Answer: C. Rotavirus
Explanation:
🔵 Rotavirus is a double-stranded RNA virus and a leading cause of infantile gastroenteritis. Its distinct wheel-shaped morphology is visible under electron microscopy, and rapid antigen tests or PCR can assist diagnosis from stool samples.
38. The best specimen for diagnosing pulmonary tuberculosis is:
A. Nasopharyngeal swab
B. Induced sputum
C. Blood culture
D. Bronchial brush
Answer: B. Induced sputum
Explanation:
🔵 Induced sputum is the preferred non-invasive specimen for detecting Mycobacterium tuberculosis. It yields higher sensitivity than spontaneous sputum and is more practical than bronchoscopy in many clinical settings.
39. Which of the following best differentiates Listeria monocytogenes from Corynebacterium species?
A. Catalase production
B. Gram stain morphology
C. Motility at room temperature
D. Beta-hemolysis on blood agar
Answer: C. Motility at room temperature
Explanation:
🔵 Listeria monocytogenes exhibits tumbling motility at room temperature (25°C), while Corynebacterium spp. are non-motile. Although both are Gram-positive rods and may appear similar under the microscope, motility testing is key for differentiation.
40. What is the most common cause of fungal meningitis in immunocompromised patients, particularly those with AIDS?
A. Aspergillus fumigatus
B. Histoplasma capsulatum
C. Cryptococcus neoformans
D. Candida albicans
Answer: C. Cryptococcus neoformans
Explanation:
🔵 Cryptococcus neoformans is an encapsulated yeast that causes life-threatening meningitis, especially in HIV/AIDS patients. India ink preparation or cryptococcal antigen tests from CSF can confirm diagnosis rapidly.
41. Which of the following bacteria is the most common contaminant of blood cultures?
A. Staphylococcus aureus
B. Streptococcus pneumoniae
C. Bacillus subtilis
D. Staphylococcus epidermidis
Answer: D. Staphylococcus epidermidis
Explanation:
🔵 Staphylococcus epidermidis is a part of normal skin flora and a frequent contaminant in blood cultures. However, in immunocompromised or catheterized patients, it may cause true bloodstream infections. Multiple positive sets or clinical correlation help distinguish contamination from true infection.
42. What media is best for isolating Bordetella pertussis from a nasopharyngeal specimen?
A. MacConkey agar
B. Chocolate agar
C. Bordet-Gengou agar
D. Thayer-Martin agar
Answer: C. Bordet-Gengou agar
Explanation:
🔵 Bordetella pertussis, the causative agent of whooping cough, requires enriched media such as Bordet-Gengou agar or Regan-Lowe medium. These media contain additives that inhibit contaminants and support the growth of this fastidious organism.
43. Which stain is most appropriate for identifying Mycobacterium tuberculosis in a sputum sample?
A. Gram stain
B. Calcofluor white
C. Acid-fast stain (Ziehl-Neelsen or Kinyoun)
D. Trichrome stain
Answer: C. Acid-fast stain (Ziehl-Neelsen or Kinyoun)
Explanation:
🔵 Mycobacterium tuberculosis has a lipid-rich cell wall containing mycolic acids, which retain the carbol fuchsin dye during acid-fast staining. Acid-fast bacilli (AFB) appear as slender red rods on a blue or green background, aiding diagnosis in sputum microscopy.
44. What is the most likely identification of a parasite seen in stool as an oval egg with a thin shell and a developing larva inside?
A. Ascaris lumbricoides
B. Hookworm
C. Strongyloides stercoralis
D. Trichuris trichiura
Answer: B. Hookworm
Explanation:
🔵 Hookworm eggs, particularly Necator americanus or Ancylostoma duodenale, are oval with a thin shell and contain a developing larva. They are common in tropical regions and can cause iron-deficiency anemia through intestinal blood loss.
45. The best initial test to differentiate between Mycobacterium kansasii and Mycobacterium tuberculosis is:
A. Niacin accumulation
B. Catalase test
C. Nitrate reduction
D. Photochromogenicity
Answer: D. Photochromogenicity
Explanation:
🔵 Mycobacterium kansasii produces yellow pigment only when exposed to light (photochromogenic), while Mycobacterium tuberculosis does not produce pigment. This test is part of the Runyon classification system used for nontuberculous mycobacteria (NTM).
46. A patient presents with chronic nasal congestion, asthma-like symptoms, and eosinophilia. Bronchoalveolar lavage reveals branching, septate hyphae. What is the most likely diagnosis?
A. Invasive aspergillosis
B. Allergic bronchopulmonary aspergillosis (ABPA)
C. Histoplasmosis
D. Pneumocystis jirovecii pneumonia
Answer: B. Allergic bronchopulmonary aspergillosis (ABPA)
Explanation:
🔵 ABPA occurs in patients with asthma or cystic fibrosis who become sensitized to Aspergillus fumigatus. The immune reaction causes bronchial obstruction, eosinophilia, elevated IgE, and transient pulmonary infiltrates. Diagnosis is clinical, supported by serology and fungal stains.
47. A CSF culture grows a yeast that is urease-positive and forms a capsule. India ink preparation reveals encapsulated organisms. What is the best confirmatory test?
A. Germ tube test
B. Cryptococcal antigen detection
C. KOH mount
D. Sabouraud agar culture
Answer: B. Cryptococcal antigen detection
Explanation:
🔵 Cryptococcus neoformans is identified by its urease positivity and thick capsule. While India ink suggests its presence, cryptococcal antigen testing (latex agglutination or lateral flow) provides high sensitivity and specificity for diagnosis, especially in CSF.
48. Which organism is oxidase-positive, grows at 42°C, and is a common cause of bacterial gastroenteritis?
A. Shigella sonnei
B. Salmonella typhi
C. Campylobacter jejuni
D. Vibrio parahaemolyticus
Answer: C. Campylobacter jejuni
Explanation:
🔵 Campylobacter jejuni is a curved, motile, oxidase-positive, Gram-negative rod that thrives at 42°C—reflecting avian body temperature. It’s a leading cause of bacterial diarrhoea worldwide, often acquired from undercooked poultry or contaminated water.
49. A 12-year-old presents with pharyngitis. The rapid antigen test is negative, but the throat culture, performed after 48 hours, shows pinpoint beta-hemolytic colonies that are PYR-positive and bacitracin-sensitive. What is the likely organism?
A. Streptococcus agalactiae
B. Streptococcus pyogenes
C. Enterococcus faecium
D. Streptococcus pneumoniae
Answer: B. Streptococcus pyogenes
Explanation:
🔵 Streptococcus pyogenes (Group A Strep) is beta-hemolytic, PYR-positive, and sensitive to bacitracin. False-negative rapid antigen tests should be confirmed with culture, as untreated strep pharyngitis can lead to rheumatic fever or glomerulonephritis.
50. A patient with AIDS presents with watery diarrhea. Modified acid-fast staining of stool shows oval, acid-fast oocysts measuring 8–10 µm. What is the likely organism?
A. Cryptosporidium parvum
B. Isospora belli
C. Cyclospora cayetanensis
D. Giardia lamblia
Answer: C. Cyclospora cayetanensis
Explanation:
🔵 Cyclospora cayetanensis produces oocysts larger than Cryptosporidium (8–10 µm vs. 4–6 µm) and appears variably acid-fast. It causes prolonged diarrhea in both immunocompetent and immunocompromised hosts, often from contaminated produce like raspberries or basil.
51. Which anaerobic, Gram-negative bacillus is commonly associated with a double-zone of hemolysis on blood agar and gas gangrene in soft tissue wounds?
A. Bacteroides fragilis
B. Prevotella melaninogenica
C. Clostridium perfringens
D. Fusobacterium nucleatum
Answer: C. Clostridium perfringens
Explanation:
🔵 Clostridium perfringens is an anaerobic, spore-forming, Gram-positive (though often Gram-variable) bacillus known for producing a double zone of hemolysis—inner complete and outer incomplete—on blood agar. It causes gas gangrene and foodborne illness via alpha toxin.
52. A liver abscess aspirate shows “anchovy paste” material. Which protozoan is the most likely cause?
A. Giardia lamblia
B. Entamoeba histolytica
C. Balantidium coli
D. Toxoplasma gondii
Answer: B. Entamoeba histolytica
Explanation:
🔵 Entamoeba histolytica is the only pathogenic amoeba among common intestinal protozoa. It causes flask-shaped ulcers in the colon and can spread hematogenously to the liver, forming abscesses filled with necrotic, reddish-brown “anchovy paste” material. Diagnosis includes serology and detection of cysts or trophozoites.
53. Which statement best describes the principle of nucleic acid amplification tests (NAATs) used in identifying Chlamydia trachomatis?
A. They detect bacterial proteins using labeled antibodies.
B. They amplify target DNA or RNA sequences for direct organism detection.
C. They measure metabolic byproducts of bacterial growth.
D. They rely on visual identification of intracellular inclusions.
Answer: B. They amplify target DNA or RNA sequences for direct organism detection.
Explanation:
🔵 NAATs (such as PCR) are highly sensitive and specific molecular tools that detect genetic material from Chlamydia trachomatis in clinical samples (e.g., urine, cervical swabs). These tests amplify target DNA or RNA, enabling early and accurate diagnosis without needing viable organisms.
54. A blood culture grows small Gram-negative coccobacilli after 5 days of incubation. The isolate is oxidase-positive, requires cysteine for growth, and is highly infectious. What is the most likely organism?
A. Brucella melitensis
B. Pasteurella multocida
C. Francisella tularensis
D. Haemophilus influenzae
Answer: C. Francisella tularensis
Explanation:
🔵 Francisella tularensis is a highly infectious, fastidious Gram-negative coccobacillus requiring cysteine-enriched media such as chocolate or buffered charcoal yeast extract (BCYE) agar. It causes tularemia and is a potential bioterrorism agent; lab exposure risk is high, requiring biosafety precautions.
56. In a mycology culture, a mold shows septate hyphae and conidia in “brush-like” structures. The colony appears green on surface and white on reverse. What is the most likely identification?
A. Penicillium species
B. Aspergillus fumigatus
C. Trichophyton rubrum
D. Fusarium species
Answer: A. Penicillium species
Explanation:
🔵 Penicillium species are common environmental molds with greenish colonies and brush-like conidiophores. While often contaminants, some species can cause infections in immunocompromised individuals. Differentiation from Aspergillus is based on conidial arrangement and colony features.
57. Which zoonotic organism is transmitted via inhalation of dried placental material or excreta from infected livestock and is known to cause culture-negative endocarditis?
A. Coxiella burnetii
B. Brucella abortus
C. Bacillus anthracis
D. Leptospira interrogans
Answer: A. Coxiella burnetii
Explanation:
🔵 Coxiella burnetii, the agent of Q fever, is an obligate intracellular organism that does not grow on standard culture media. It’s notorious for causing culture-negative endocarditis in patients with pre-existing heart conditions. Transmission occurs through the inhalation of contaminated aerosols, especially those from the birthing fluids of livestock.
58. What is the most appropriate stain to visualize the capsule of Cryptococcus neoformans in CSF?
A. Calcofluor white
B. India ink
C. Acid-fast stain
D. Trichrome stain
Answer: B. India ink
Explanation:
🔵 India ink is a negative staining technique that outlines the polysaccharide capsule of Cryptococcus neoformans, appearing as a clear halo around the yeast against a dark background. Though less sensitive than antigen detection, it’s useful in resource-limited settings and classic teaching.
59. Which virus is most commonly associated with congenital infection leading to sensorineural hearing loss in newborns?
A. Rubella virus
B. Cytomegalovirus
C. Herpes simplex virus type 2
D. Varicella-zoster virus
Answer: B. Cytomegalovirus
Explanation:
🔵 Cytomegalovirus (CMV) is the most common congenital viral infection worldwide. Transmission occurs transplacentally, and although many infants are asymptomatic, sensorineural hearing loss is a common long-term sequela. Diagnosis involves PCR or viral culture from urine/saliva.
60. An isolate from a wound shows swarming motility on blood agar and produces hydrogen sulfide (H₂S) on triple sugar iron (TSI) agar. What is the likely organism?
A. Salmonella enterica
B. Proteus mirabilis
C. Escherichia coli
D. Shigella flexneri
Answer: B. Proteus mirabilis
Explanation:
🔵 Proteus mirabilis is a member of Enterobacterales known for its swarming motility and H₂S production. These features help differentiate it from other Gram-negative rods. It also deaminates phenylalanine and is urease-positive, often associated with urinary tract infections.
61. What is the principle of MALDI-TOF MS used for rapid microbial identification?
A. Separation of nucleic acids via gel electrophoresis
B. Detection of metabolic end-products by color change
C. Analysis of protein mass spectra via ionization
D. Hybridization of rRNA probes with target sequences
Answer: C. Analysis of protein mass spectra via ionization
Explanation:
🔵 Matrix-Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry (MALDI-TOF MS) identifies microorganisms based on unique protein fingerprinting. The laser ionizes microbial proteins, which are then measured by time-of-flight; the resulting spectrum is matched to a reference database for ID.
62. A neutropenic patient develops a rapidly progressing sinus infection. Biopsy reveals broad, ribbon-like, non-septate hyphae branching at right angles. Which organism is the most likely cause?
A. Aspergillus fumigatus
B. Candida albicans
C. Rhizopus species
D. Fusarium solani
Answer: C. Rhizopus species
Explanation:
🔵 Rhizopus, a member of the order Mucorales, causes mucormycosis—especially in immunocompromised or diabetic ketoacidotic patients. These molds display broad, aseptate hyphae with right-angle branching. Rapid surgical and antifungal treatment is crucial due to its aggressive angioinvasive nature.
63. Which virus is diagnosed by detecting heterophile antibodies in the Monospot test?
A. Epstein-Barr virus
B. Cytomegalovirus
C. Human herpesvirus 6
D. Parvovirus B19
Answer: A. Epstein-Barr virus
Explanation:
🔵 Epstein-Barr virus (EBV) causes infectious mononucleosis, often confirmed via Monospot test, which detects heterophile antibodies that agglutinate horse or sheep RBCs. Atypical lymphocytosis is also a common lab finding. Note that CMV can mimic mono but lacks heterophile antibodies.
64. A blood culture grows Gram-negative rods that are oxidase-positive, indole-positive, and have a fruity odor. Colonies appear blue-green on MacConkey agar. Which organism is most likely?
A. Klebsiella pneumoniae
B. Escherichia coli
C. Pseudomonas aeruginosa
D. Enterobacter cloacae
Answer: C. Pseudomonas aeruginosa
Explanation:
🔵 Pseudomonas aeruginosa is a non-fermenting, oxidase-positive bacillus known for its grape-like odor and blue-green pigment (pyocyanin). It is a common cause of bacteremia in hospitalized and immunocompromised patients, especially those with burns or on ventilators.
65. Which laboratory method is best for differentiating herpes simplex virus types 1 and 2?
A. Tzanck smear
B. Viral culture
C. NAAT (PCR-based testing)
D. Complement fixation
Answer: C. NAAT (PCR-based testing)
Explanation:
🔵 PCR or other nucleic acid amplification tests (NAATs) are highly specific and sensitive for distinguishing HSV-1 from HSV-2. Tzanck smears may reveal multinucleated giant cells but cannot differentiate types. Culture is slower and less reliable, especially in healing lesions.
66. What is the correct Biosafety Level (BSL) for working with Mycobacterium tuberculosis in the laboratory?
A. BSL-1
B. BSL-2
C. BSL-3
D. BSL-4
Answer: C. BSL-3
Explanation:
🔵 BSL-3 laboratories are designed for aerosol-transmitted, potentially lethal pathogens, such as Mycobacterium tuberculosis. These facilities require negative air pressure, HEPA filtration, and protective equipment to prevent lab-acquired infections.
67. Which anaerobic, Gram-positive bacillus is associated with pseudomembranous colitis and is diagnosed by detecting toxin A/B in stool?
A. Clostridium botulinum
B. Clostridium perfringens
C. Clostridium difficile
D. Clostridium septicum
Answer: C. Clostridium difficile
Explanation:
🔵 Clostridium difficile is the causative agent of antibiotic-associated colitis. Diagnosis typically involves detection of toxins A and B in stool via EIA or PCR. The presence of pseudomembranes on colonoscopy further confirms the diagnosis. Spores are resistant to many disinfectants.
68. Which parasite demonstrates a lateral spine on its egg and is endemic in areas with freshwater snails?
A. Schistosoma haematobium
B. Schistosoma mansoni
C. Fasciola hepatica
D. Clonorchis sinensis
Answer: B. Schistosoma mansoni
Explanation:
🔵 Schistosoma mansoni eggs exhibit a lateral spine and are found in stool samples. Transmission occurs when skin comes into contact with cercariae-infested freshwater in endemic regions. Intermediate hosts are snails. In contrast, S. haematobium eggs have a terminal spine and appear in urine.
69. Which of the following is most responsible for methicillin resistance in Staphylococcus aureus?
A. Beta-lactamase production
B. mecA gene encoding PBP2a
C. Altered porin channels
D. Efflux pump activation
Answer: B. mecA gene encoding PBP2a
Explanation:
🔵 The mecA gene encodes an altered penicillin-binding protein (PBP2a) that reduces affinity for beta-lactams, conferring resistance to methicillin and other related antibiotics. This gene is detected by PCR and is a key marker in identifying MRSA strains.
70. A transplant patient develops retinitis. PCR from vitreous fluid is positive for a large, enveloped, DNA virus with “owl’s eye” inclusion bodies. What is the likely pathogen?
A. Herpes simplex virus
B. Cytomegalovirus
C. Epstein-Barr virus
D. Varicella-zoster virus
Answer: B. Cytomegalovirus
Explanation:
🔵 CMV retinitis is a serious opportunistic infection in immunocompromised hosts, particularly organ transplant recipients or AIDS patients. Histologic hallmark is intranuclear “owl’s eye” inclusions, and PCR aids in confirmation. Immediate antiviral therapy is critical to prevent blindness.
71. A Gram-negative diplococcus grows on Thayer-Martin agar and is identified as oxidase-positive and glucose-fermenting only. What additional test confirms pathogenicity in males with urethritis?
A. Urease test
B. Beta-lactamase test
C. Coagulase test
D. Superoxol test (30% H₂O₂ reaction)
Answer: D. Superoxol test (30% H₂O₂ reaction)
Explanation:
🔵 The Superoxol test, using 30% hydrogen peroxide, causes an immediate vigorous bubbling reaction in Neisseria gonorrhoeae but not in other Neisseria spp. It supports identification in clinical isolates, especially when combined with glucose-only fermentation and positive oxidase activity.
72. Which test is most useful for rapid detection of Histoplasma capsulatum in disseminated infections, especially in immunocompromised patients?
A. Blood culture on Sabouraud agar
B. Direct KOH mount of skin lesion
C. Urine antigen test
D. India ink stain of CSF
Answer: C. Urine antigen test
Explanation:
🔵 Histoplasma capsulatum is a dimorphic fungus endemic in certain U.S. regions. In disseminated histoplasmosis, especially in HIV-positive patients, urine antigen detection offers high sensitivity and rapid diagnosis. Cultures are slow and often yield results too late for early treatment.
73. Which of the following organisms is a biosafety concern due to its ability to be aerosolized and used as a bioterrorism agent, and requires BSL-3 precautions?
A. Brucella melitensis
B. Yersinia enterocolitica
C. Campylobacter jejuni
D. Pasteurella multocida
Answer: A. Brucella melitensis
Explanation:
🔵 Brucella melitensis is an intracellular, Gram-negative coccobacillus that can aerosolize easily in lab settings. It causes undulant fever and poses a high risk of lab-acquired infection. Culturing Brucella requires BSL-3 precautions and extended incubation.
74. A thick and thin blood smear reveals ring forms inside RBCs, some with multiple rings per cell and applique forms. Travel history includes sub-Saharan Africa. What is the most likely parasite?
A. Plasmodium vivax
B. Plasmodium ovale
C. Plasmodium falciparum
D. Babesia microti
Answer: C. Plasmodium falciparum
Explanation:
🔵 Plasmodium falciparum shows multiple ring forms per RBC and “appliqué” forms, which appear on the edge of the cell. It’s the most lethal form of malaria, prevalent in sub-Saharan Africa. It does not form schizonts in peripheral blood due to sequestration.
75. Resistance to vancomycin in Enterococcus faecium is most commonly due to:
A. Altered porin channels
B. beta-lactamase production
C. VanA gene encoding altered peptidoglycan precursor
D. mecA gene encoding PBP2a
Answer: C. VanA gene encoding an altered peptidoglycan precursor
Explanation:
🔵 Vancomycin-resistant Enterococcus (VRE) typically carries the VanA gene, which modifies the terminal D-Ala-D-Ala in peptidoglycan to D-Ala-D-Lac, significantly reducing vancomycin binding. This resistance is of major concern in nosocomial infections and requires contact isolation.
76. Which virus can integrate its genome into host DNA and is strongly associated with hepatocellular carcinoma?
A. Hepatitis A virus
B. Hepatitis B virus
C. Hepatitis C virus
D. Epstein-Barr virus
Answer: B. Hepatitis B virus
Explanation:
🔵 Hepatitis B virus (HBV) is a DNA virus that uses reverse transcription and can integrate into the host genome. Chronic HBV infection increases the risk for hepatocellular carcinoma, particularly when viral replication is unchecked over years.
77. Which fungal agent is urease-positive, phenol oxidase-positive, and produces melanin on birdseed (niger seed) agar?
A. Candida glabrata
B. Cryptococcus neoformans
C. Histoplasma capsulatum
D. Blastomyces dermatitidis
Answer: B. Cryptococcus neoformans
Explanation:
🔵 Cryptococcus neoformans produces melanin pigment due to phenol oxidase activity when grown on birdseed agar, resulting in brown colonies. It is also urease-positive and encapsulated, which aids in identification. This fungus is especially significant in AIDS patients presenting with meningitis.
78. A patient returning from Southeast Asia presents with a chronic ulcer with black eschar. Gram stain of culture reveals safety-pin–shaped Gram-negative rods. What is the most likely organism?
A. Pasteurella multocida
B. Yersinia pestis
C. Burkholderia pseudomallei
D. Francisella tularensis
Answer: C. Burkholderia pseudomallei
Explanation:
🔵 Burkholderia pseudomallei, the causative agent of melioidosis, is endemic in Southeast Asia and northern Australia. It appears as a bipolar (“safety-pin”) Gram-negative rod, often causing skin ulcers, pneumonia, or abscesses. Culturing requires biosafety precautions due to aerosol risk.
79. Which stool concentration technique is best for detecting protozoan cysts and helminth eggs with high sensitivity?
A. Wet mount
B. Trichrome stain
C. Formalin-ethyl acetate concentration
D. Modified acid-fast stain
Answer: C. Formalin-ethyl acetate concentration
Explanation:
🔵 Formalin-ethyl acetate concentration is a sedimentation technique that improves detection of protozoan cysts and helminth eggs by concentrating them at the bottom of the tube. It’s one of the standard initial steps in routine ova and parasite (O&P) examination.
80. Which hepatitis virus is defective and requires co-infection with another hepatitis virus to replicate?
A. Hepatitis A virus
B. Hepatitis B virus
C. Hepatitis C virus
D. Hepatitis D virus
Answer: D. Hepatitis D virus
Explanation:
🔵 Hepatitis D virus (HDV) is a defective RNA virus that depends on Hepatitis B virus (HBV) for envelope proteins. Co-infection or superinfection with HBV can lead to severe liver disease. Detection involves HDV RNA or anti-HDV antibodies.
81. Which selective medium is best used for isolating Campylobacter jejuni from stool samples?
A. Thiosulfate-citrate-bile salts-sucrose (TCBS) agar
B. Hektoen enteric agar
C. MacConkey agar
D. Campy-BAP (Campylobacter blood agar with antibiotics)
Answer: D. Campy-BAP (Campylobacter blood agar with antibiotics)
Explanation:
🔵 Campy-BAP is a selective medium enriched with blood and antibiotics (like vancomycin, polymyxin B, and trimethoprim) that inhibit normal gut flora and support growth of Campylobacter jejuni. Incubation requires microaerophilic conditions at 42°C, mimicking avian temperatures.
82. Which zoonotic virus is transmitted by fruit bats and causes severe hemorrhagic fever with a high mortality rate?
A. Hantavirus
B. Lassa virus
C. Nipah virus
D. Dengue virus
Answer: C. Nipah virus
Explanation:
🔵 Nipah virus is a paramyxovirus transmitted by fruit bats (flying foxes) and has caused fatal outbreaks in Southeast Asia. It leads to encephalitis and hemorrhagic manifestations, with high person-to-person transmission risk. No specific treatment exists, and containment relies on isolation and supportive care.
83. A fungemia case in a neutropenic patient reveals yeast-like cells with pseudohyphae and terminal chlamydospores on cornmeal agar. What is the most likely organism?
A. Candida tropicalis
B. Candida glabrata
C. Candida albicans
D. Cryptococcus gattii
Answer: C. Candida albicans
Explanation:
🔵 Candida albicans produces pseudohyphae, blastoconidia, and terminal chlamydospores on cornmeal agar, which helps differentiate it from other non-albicans Candida species. It is a leading cause of fungemia, especially in immunocompromised or catheterized patients.
84. Which parasite uses snails as intermediate hosts and migrates through the lungs during its human life cycle, often causing rusty sputum and chronic cough?
A. Paragonimus westermani
B. Clonorchis sinensis
C. Fasciola hepatica
D. Schistosoma haematobium
Answer: A. Paragonimus westermani
Explanation:
🔵 Paragonimus westermani, the lung fluke, infects humans through undercooked crustaceans. It migrates through the lungs, leading to chronic cough and bloody (rusty) sputum. Diagnosis is by detecting eggs in sputum or stool. Snails serve as the first intermediate host in its complex life cycle.
85. Which bacterial toxin inactivates elongation factor-2 (EF-2), halting host cell protein synthesis and causing pseudomembrane formation?
A. Shiga toxin
B. Diphtheria toxin
C. Tetanospasmin
D. Botulinum toxin
Answer: B. Diphtheria toxin
Explanation:
🔵 Corynebacterium diphtheriae produces diphtheria toxin, which inactivates EF-2 via ADP-ribosylation, halting protein synthesis. This leads to cell death and pseudomembrane formation in the pharynx, a hallmark of diphtheria. Toxigenicity is confirmed with the Elek test or PCR.
86. Which advanced diagnostic tool allows simultaneous detection of multiple pathogens using DNA probes and is often used in syndromic panels (e.g., respiratory or GI panels)?
A. MALDI-TOF MS
B. NAAT (singleplex PCR)
C. Lateral flow assay
D. Multiplex PCR
Answer: D. Multiplex PCR
Explanation:
🔵 Multiplex PCR amplifies multiple DNA targets simultaneously, allowing rapid identification of several pathogens in a single sample. It’s commonly used in syndromic panels for respiratory infections, meningitis, or gastrointestinal pathogens—enhancing diagnostic speed and accuracy in clinical microbiology.
87. Which serologic test is most useful for diagnosing invasive aspergillosis in neutropenic patients?
A. India ink stain
B. (1→3)-β-D-glucan assay
C. Galactomannan antigen detection
D. Anti-Aspergillus IgM ELISA
Answer: C. Galactomannan antigen detection
Explanation:
🔵 Galactomannan, a component of the Aspergillus cell wall, is released during invasive infection. The galactomannan antigen test is especially helpful in neutropenic patients, providing early detection through serum or bronchoalveolar lavage (BAL) fluid. It’s often paired with imaging and other fungal biomarkers.
88. What is the most likely hepatitis virus to result in chronic infection and hepatocellular carcinoma in perinatally infected infants?
A. Hepatitis A
B. Hepatitis B
C. Hepatitis C
D. Hepatitis E
Answer: B. Hepatitis B
Explanation:
🔵 Hepatitis B virus (HBV) transmitted from mother to infant during birth leads to chronic infection in up to 90% of cases. These infants are at a significantly higher risk for cirrhosis and liver cancer later in life. Early vaccination and HBIG prophylaxis are essential.
89. Which species of Mycobacterium grows rapidly (<7 days) and is commonly associated with skin and soft tissue infections after trauma or surgery?
A. Mycobacterium tuberculosis
B. Mycobacterium kansasii
C. Mycobacterium fortuitum
D. Mycobacterium avium-intracellulare
Answer: C. Mycobacterium fortuitum
Explanation:
🔵 Mycobacterium fortuitum is a rapid-growing nontuberculous mycobacterium (NTM). It causes post-surgical wound infections, catheter-related sepsis, or abscesses. It can grow on routine media within 7 days, unlike M. tuberculosis which takes several weeks.
90. In the laboratory, which activity poses the highest aerosol risk when handling Brucella species?
A. Gram staining
B. Inoculating a blood culture
C. Subculturing from a positive bottle
D. Disposing of media plates
Answer: C. Subculturing from a positive bottle
Explanation:
🔵 Brucella spp. pose a high risk of aerosol transmission, especially during subculturing, when small droplets can be released unknowingly. Due to its low infectious dose, all manipulation should be performed in a biosafety cabinet under BSL-3 precautions.
91. A blood smear reveals extracellular and intraerythrocytic ring-like organisms in a patient with febrile illness. The patient has no recent travel but had a tick bite. What is the likely pathogen?
A. Plasmodium vivax
B. Trypanosoma cruzi
C. Babesia microti
D. Leishmania donovani
Answer: C. Babesia microti
Explanation:
🔵 Babesia microti, transmitted by Ixodes ticks, is endemic in parts of the U.S. (e.g., Northeast). It mimics malaria on blood smears but may form Maltese cross tetrads. Unlike Plasmodium, there’s no travel history to endemic areas and no pigment in RBCs.
92. A mold grown from a corneal scraping produces spindle-shaped macroconidia and is resistant to many antifungals. What is the likely organism?
A. Trichophyton rubrum
B. Microsporum canis
C. Fusarium species
D. Aspergillus niger
Answer: C. Fusarium species
Explanation:
🔵 Fusarium produces banana- or canoe-shaped macroconidia and is often associated with eye infections, especially keratitis. It is inherently resistant to many antifungal agents, making infections difficult to treat. Bloodstream infections can occur in neutropenic patients.
93. Which type of infectious agent lacks nucleic acid and causes spongiform encephalopathy in humans?
A. Retrovirus
B. Protozoan
C. Prion
D. Viroid
Answer: C. Prion
Explanation:
🔵 Prions are misfolded proteins that induce abnormal folding in normal host proteins, particularly in the CNS. They lack DNA or RNA and cause diseases like Creutzfeldt-Jakob disease (CJD). They are highly resistant to sterilization and inactivation.
94. In molecular diagnostics, what does a “Ct” value represent in real-time PCR?
A. The concentration of DNA template
B. The melting temperature of the probe
C. The cycle at which fluorescence crosses the threshold
D. The number of amplicons generated
Answer: C. The cycle at which fluorescence crosses the threshold
Explanation: Ct (Cycle threshold) is the number of cycles required for the fluorescent signal to exceed background. Lower Ct values indicate higher amounts of target nucleic acid, and it’s commonly used in quantitative PCR diagnostics, like viral load monitoring.
95. Which organism is most associated with laboratory-acquired infections due to its ability to infect via aerosols and require extreme caution in handling?
A. Neisseria gonorrhoeae
B. Francisella tularensis
C. Candida glabrata
D. Mycoplasma pneumoniae
Answer: B. Francisella tularensis
Explanation:
🔵 Francisella tularensis is highly infectious, requiring as few as 10 organisms to cause tularemia. It spreads via aerosols, direct contact, or ingestion, making it one of the most dangerous agents in lab environments. BSL-3 practices are required.
96. A stool sample reveals oval, operculated eggs with a “shouldered” appearance. The patient reports eating raw freshwater fish. What is the most likely parasite?
A. Fasciola hepatica
B. Clonorchis sinensis
C. Diphyllobothrium latum
D. Paragonimus westermani
Answer: B. Clonorchis sinensis
Explanation:
🔵 Clonorchis sinensis is the Chinese liver fluke, transmitted via undercooked freshwater fish. Its eggs are oval with a prominent operculum and shouldered edges, and are found in stool. It infects the biliary tract, potentially leading to cholangitis and cholangiocarcinoma.
97. A transplant recipient presents with hemorrhagic cystitis. PCR detects a DNA virus in urine that lacks an envelope and has a circular double-stranded genome. What is the likely pathogen?
A. Cytomegalovirus
B. JC virus
C. BK virus
D. Adenovirus
Answer: C. BK virus
Explanation:
🔵 BK virus, a polyomavirus, causes hemorrhagic cystitis and nephropathy in renal and bone marrow transplant patients. It is a non-enveloped, circular dsDNA virus, detectable by PCR in blood or urine. JC virus, another polyomavirus, causes progressive multifocal leukoencephalopathy (PML).
98. Which mycotoxin-producing fungus is a major concern in improperly stored grains and nuts, particularly for hepatocellular carcinoma development?
A. Penicillium marneffei
B. Aspergillus flavus
C. Fusarium oxysporum
D. Candida krusei
Answer: B. Aspergillus flavus
Explanation:
🔵 Aspergillus flavus produces aflatoxins, potent hepatocarcinogens. Contamination of grains, nuts, and corn in warm, humid storage conditions allows toxin accumulation. Long-term consumption is linked to liver cancer, especially in populations with chronic hepatitis B.
99. What is the best confirmatory test for identifying Mycoplasma pneumoniae in a respiratory specimen?
A. Gram stain
B. Cold agglutinin titer
C. Culture on blood agar
D. PCR for specific DNA sequences
Answer: D. PCR for specific DNA sequences
Explanation:
🔵 Mycoplasma pneumoniae lacks a cell wall and does not stain with Gram stain or grow well on standard media. PCR is highly specific and sensitive, making it the preferred confirmatory test for respiratory specimens. Cold agglutinins are nonspecific.
100. Which diagnostic feature best distinguishes Trichophyton tonsurans from other dermatophytes causing tinea capitis?
A. Production of yellow pigment on Sabouraud agar
B. Perforation of hair shafts (endothrix pattern)
C. Macroconidia with spiral hyphae
D. Urease-negative colonies
Answer: B. Perforation of hair shafts (endothrix pattern)
Explanation:
🔵 Trichophyton tonsurans is the most common cause of tinea capitis in children and invades the hair shaft internally (endothrix). Unlike Microsporum spp., it doesn’t fluoresce under Wood’s lamp. Identification may include hair perforation test and colony morphology.
R E S O U R C E S :
- Atlas of Clinical Microbiology by M. J. Z. M. Y. Y. Hsiao
- Clinical Microbiology Reviews by American Society for Microbiology
- Microbiology: Principles and Explorations by Jacquelyn G. Black
- Molecular and Clinical Dermatology: Chlamydia and Rickettsia by D. R. Foster
- Manual of Clinical Microbiology by Karen C. Carroll, et al.