Microbiology MCQs – Part 1 (Q1–Q25)

Q1. The primary structural component of most bacterial cell walls is:

A) Cellulose
B) Chitin
C) Peptidoglycan
D) Ergosterol

✅ Answer: C) Peptidoglycan
Explanation: Peptidoglycan (murein) gives bacteria rigidity and shape.

A) Cellulose → plants/algae.
B) Chitin → fungi exoskeletons/arthropods.
D) Ergosterol → fungal membranes.

Q2. Which of the following lacks a cell wall?

A) Mycoplasma pneumoniae
B) Staphylococcus aureus
C) Escherichia coli
D) Bacillus subtilis

✅ Answer: A) Mycoplasma pneumoniae
Explanation: Mycoplasma spp. do not have cell walls; sterols stabilize their membranes.

B–D) Typical bacteria with peptidoglycan walls.

Q3. Gram staining differentiates bacteria based on:

A) Capsule thickness
B) Spore presence
C) Cell wall peptidoglycan architecture
D) Flagellar number

✅ Answer: C) Cell wall peptidoglycan architecture
Explanation: Gram-positive retain crystal violet due to thick peptidoglycan; Gram-negative have a thin layer and outer membrane.

A, B, D) Not the basis of Gram reaction.

Q4. Lipopolysaccharide (LPS) is characteristic of:

A) Gram-positive bacteria
B) Gram-negative bacteria
C) Mycobacteria only
D) Archaea

✅ Answer: B) Gram-negative bacteria
Explanation: LPS is in the outer membrane of Gram-negatives; lipid A is endotoxin.

A) No outer membrane.
C) Mycobacteria have mycolic acids.
D) Archaea lack LPS.

Q5. Bacterial endospores are primarily for:

A) Reproduction
B) Genetic recombination
C) Survival under harsh conditions
D) Energy storage

✅ Answer: C) Survival under harsh conditions
Explanation: Endospores (e.g., Bacillus, Clostridium) resist heat, desiccation, chemicals.

A, B, D) Not reproductive, recombination, or storage structures.

Q6. Which enzyme unwinds DNA ahead of the replication fork in bacteria?

A) DNA polymerase I
B) DNA gyrase (topoisomerase II)
C) RNA polymerase
D) Reverse transcriptase

✅ Answer: B) DNA gyrase
Explanation: Gyrase relieves supercoils during replication.

A) Fills gaps/repair.
C) Transcription, not replication.
D) Retroviruses, not bacteria.

Q7. Which phase of bacterial growth shows antibiotic β-lactam maximal effectiveness?

A) Lag phase
B) Log (exponential) phase
C) Stationary phase
D) Death phase

✅ Answer: B) Log (exponential) phase
Explanation: β-lactams target actively synthesizing cell walls.

A, C, D) Reduced peptidoglycan synthesis → lower efficacy.

Q8. A facultative anaerobe:

A) Grows only without oxygen
B) Requires oxygen
C) Grows with or without oxygen
D) Is killed by oxygen

✅ Answer: C) Grows with or without oxygen
Explanation: Uses O₂ if present (more ATP) but can ferment anaerobically.

A) Obligate anaerobe.
B) Obligate aerobe.
D) Obligate anaerobe (O₂ toxic).

Q9. Koch’s postulates were designed to:

A) Prove vaccination efficacy
B) Link a specific microbe to a specific disease
C) Classify bacteria by morphology
D) Determine antibiotic susceptibility

✅ Answer: B) Link a microbe to a disease
Explanation: They establish causation in infectious disease.

A, C, D) Not the purpose.

Q10. Which medium is selective for Gram-negative rods and differential for lactose fermentation?

A) Blood agar
B) MacConkey agar
C) Chocolate agar
D) Sabouraud agar

✅ Answer: B) MacConkey agar
Explanation: Bile salts/crystal violet inhibit Gram-positives; lactose fermenters → pink colonies.

A) Enriched, not selective.
C) Fastidious organisms (e.g., Neisseria).
D) Fungi.

Q11. Catalase-positive organisms:

A) Convert H₂O₂ to water and O₂
B) Reduce nitrate to nitrite
C) Hydrolyze urea
D) Split esculin in bile

✅ Answer: A) Convert H₂O₂ to water and O₂
Explanation: Catalase neutralizes peroxides.

B) Nitrate reductase.
C) Urease.
D) Bile esculin test (Enterococcus group D).

Q12. A positive coagulase test strongly suggests:

A) Streptococcus pyogenes
B) Staphylococcus aureus
C) Staphylococcus epidermidis
D) Enterococcus faecalis

✅ Answer: B) Staphylococcus aureus
Explanation: Coagulase+ differentiates S. aureus from other staphylococci.

C) Coagulase–.
A, D) Not staphylococci.

Q13. Acid-fast staining detects:

A) Capsule
B) Mycolic acid–rich cell walls
C) Flagella
D) Endospores

✅ Answer: B) Mycolic acid–rich cell walls
Explanation: Ziehl–Neelsen stains Mycobacterium/Nocardia due to mycolic acids.

A, C, D) Different specialized stains.

Q14. Which viral genome can be directly translated by host ribosomes?

A) dsDNA
B) (+) ssRNA
C) (–) ssRNA
D) dsRNA

✅ Answer: B) (+) ssRNA
Explanation: Positive-sense RNA acts as mRNA upon entry.

A) Requires transcription.
C) Needs RNA-dependent RNA polymerase to make +RNA.
D) Requires specialized enzymes.

Q15. An enveloped virus is generally:

A) More resistant to desiccation and detergents
B) Less resistant to desiccation and detergents
C) Unaffected by ether/solvents
D) Unable to infect animals

✅ Answer: B) Less resistant
Explanation: Lipid envelopes are labile to drying, heat, detergents, organic solvents.

A, C) Opposite.
D) False.

Q16. Reverse transcriptase is essential for:

A) Orthomyxoviruses
B) Picornaviruses
C) Retroviruses
D) Reoviruses

✅ Answer: C) Retroviruses
Explanation: RT converts RNA → DNA (e.g., HIV).

A) Negative-sense RNA; no RT.
B) Positive-sense RNA; no RT.
D) dsRNA; no RT.

Q17. Fungal cell membranes contain predominantly:

A) Cholesterol
B) Ergosterol
C) Hopanoids
D) Stigmasterol

✅ Answer: B) Ergosterol
Explanation: Target of azoles and amphotericin B.

A) Animal cells.
C) Some bacteria.
D) Plant sterol.

Q18. India ink preparation is classically used to visualize:

A) Candida albicans hyphae
B) Cryptococcus neoformans capsule
C) Aspergillus conidiophores
D) Dermatophyte spores in skin scrapings

✅ Answer: B) Cryptococcus neoformans capsule
Explanation: Negative staining highlights the thick polysaccharide capsule.

A, C, D) Seen by other stains/KOH mounts.

Q19. The definitive host for Plasmodium is:

A) Human
B) Anopheles mosquito
C) Tsetse fly
D) Sandfly

✅ Answer: B) Anopheles mosquito
Explanation: Sexual reproduction occurs in mosquito; humans are intermediate (asexual).

C) Trypanosoma vector.
D) Leishmania vector.

Q20. Which immunoglobulin is first produced in a primary immune response?

A) IgA
B) IgM
C) IgG
D) IgE

✅ Answer: B) IgM
Explanation: Pentameric IgM appears early; IgG predominates later and in memory responses.

A, D) Specialized roles (mucosa/allergy).

Q21. The classical pathway of complement is primarily activated by:

A) Mannose-binding lectin binding to microbes
B) Spontaneous C3 hydrolysis
C) Antigen–antibody complexes (IgG/IgM)
D) Bacterial endotoxin directly

✅ Answer: C) Antigen–antibody complexes
Explanation: Classical pathway is antibody-dependent.

A) Lectin pathway.
B) Alternative pathway tickover.
D) Alternative pathway trigger.

Q22. A superantigen characteristically:

A) Inhibits MHC presentation
B) Bridges MHC II and TCR Vβ to polyclonally activate T cells
C) Only activates B cells
D) Is synonymous with endotoxin

✅ Answer: B) Bridges MHC II and TCR Vβ
Explanation: Causes massive cytokine release (e.g., TSST-1).

A) It bypasses specificity, not inhibit.
C) T cells, not B cells.
D) Different entity.

Q23. Minimum inhibitory concentration (MIC) is:

A) Lowest drug concentration that kills 99.9% bacteria
B) Lowest concentration that inhibits visible growth
C) Highest safe therapeutic dose
D) Time to reduce bacterial count by half

✅ Answer: B) Lowest concentration that inhibits visible growth
Explanation: MIC reflects inhibition;

A) Defines MBC (minimum bactericidal concentration).
C, D) Not MIC definitions.

Q24. Which antimicrobial inhibits peptidoglycan cross-linking by binding PBPs?

A) Vancomycin
B) Penicillin G
C) Bacitracin
D) Polymyxin B

✅ Answer: B) Penicillin G
Explanation: β-lactams bind penicillin-binding proteins (transpeptidases) → block cross-linking.

A) Binds D-Ala-D-Ala termini (blocks polymerization).
C) Blocks bactoprenol carrier.
D) Disrupts Gram– outer membranes.

Q25. Extended-spectrum β-lactamases (ESBLs) confer resistance to:

A) Carbapenems
B) 3rd-generation cephalosporins
C) Glycopeptides
D) Aminoglycosides

✅ Answer: B) 3rd-generation cephalosporins
Explanation: ESBLs hydrolyze 3rd/4th-gen cephalosporins and monobactams; carbapenems remain active (unless carbapenemase present).

A) Not ESBL (that’s KPC/NDM etc.).
C, D) Different targets.

Microbiology MCQs – Part 2 (Q26–Q50)

**Q26. The vector for transmission of Yersinia pestis (plague) is:**

A) Anopheles mosquito
B) Rat flea (Xenopsylla cheopis)
C) Tsetse fly
D) Sandfly

✅ Answer: B) Rat flea (Xenopsylla cheopis)
Explanation: Plague is transmitted by the bite of infected rat fleas.

A) Malaria vector.
C) Trypanosomiasis.
D) Leishmaniasis.

Q27. Which bacterial structure is responsible for motility?

A) Capsule
B) Pili
C) Flagella
D) Fimbriae

✅ Answer: C) Flagella
Explanation: Flagella provide locomotion.

A) Capsule → protection.
B, D) Pili/fimbriae → adhesion, conjugation.

Q28. Which organism is associated with peptic ulcer disease?

A) Helicobacter pylori
B) Salmonella Typhi
C) Shigella dysenteriae
D) Clostridium difficile

✅ Answer: A) Helicobacter pylori
Explanation: Produces urease, neutralizes gastric acid, causes gastritis/ulcers.

Q29. Which pathogen causes gas gangrene?

A) Clostridium perfringens
B) Bacillus anthracis
C) Staphylococcus aureus
D) Streptococcus pyogenes

✅ Answer: A) Clostridium perfringens
Explanation: An anaerobic spore-forming bacillus producing exotoxins.

Q30. Which organism produces diphtheria toxin?

A) Corynebacterium diphtheriae
B) Streptococcus pneumoniae
C) Haemophilus influenzae
D) Klebsiella pneumoniae

✅ Answer: A) Corynebacterium diphtheriae
Explanation: Diphtheria toxin inhibits EF-2 → stops protein synthesis.

Q31. A common cause of nosocomial urinary tract infections (UTI) is:

A) Pseudomonas aeruginosa
B) Escherichia coli
C) Proteus mirabilis
D) Enterococcus faecalis

✅ Answer: B) Escherichia coli
Explanation: E. coli is the leading cause of UTI, including hospital-acquired cases.

Q32. Which organism is associated with food poisoning due to reheated rice?

A) Salmonella enteritidis
B) Clostridium botulinum
C) Bacillus cereus
D) Listeria monocytogenes

✅ Answer: C) Bacillus cereus
Explanation: Produces heat-stable toxin in fried/reheated rice → vomiting.

Q33. Which bacterium is an obligate intracellular parasite?

A) Mycobacterium tuberculosis
B) Rickettsia rickettsii
C) Clostridium tetani
D) Streptococcus pyogenes

✅ Answer: B) Rickettsia rickettsii
Explanation: Rickettsiae must grow inside host cells.

Q34. Which protozoan causes sleeping sickness?

A) Giardia lamblia
B) Trypanosoma brucei
C) Entamoeba histolytica
D) Trichomonas vaginalis

✅ Answer: B) Trypanosoma brucei
Explanation: Transmitted by tsetse fly, causes African sleeping sickness.

Q35. Which parasite causes amoebic dysentery?

A) Giardia lamblia
B) Entamoeba histolytica
C) Balantidium coli
D) Cryptosporidium parvum

✅ Answer: B) Entamoeba histolytica
Explanation: Causes bloody diarrhea with flask-shaped ulcers in intestine.

Q36. Which protozoan causes malaria?

A) Trypanosoma
B) Leishmania
C) Plasmodium
D) Giardia

✅ Answer: C) Plasmodium
Explanation: Different species (P. falciparum, P. vivax, etc.) transmitted by Anopheles mosquito.

**Q37. The infective stage of Plasmodium for humans is:**

A) Sporozoite
B) Merozoite
C) Gametocyte
D) Ookinete

✅ Answer: A) Sporozoite
Explanation: Sporozoites enter human bloodstream during mosquito bite.

Q38. Kala-azar (visceral leishmaniasis) is caused by:

A) Leishmania donovani
B) Plasmodium falciparum
C) Trypanosoma cruzi
D) Entamoeba histolytica

✅ Answer: A) Leishmania donovani
Explanation: Transmitted by sandfly bite, infects spleen, liver, bone marrow.

Q39. Which helminth causes elephantiasis?

A) Wuchereria bancrofti
B) Ascaris lumbricoides
C) Taenia solium
D) Trichinella spiralis

✅ Answer: A) Wuchereria bancrofti
Explanation: Transmitted by mosquito bite; blocks lymphatics causing lymphoedema/elephantiasis.

Q40. Which parasite causes pork tapeworm infection?

A) Taenia saginata
B) Taenia solium
C) Diphyllobothrium latum
D) Hymenolepis nana

✅ Answer: B) Taenia solium
Explanation: Humans infected by eating undercooked pork.

Q41. Which worm infection is associated with perianal itching at night?

A) Ascaris lumbricoides
B) Enterobius vermicularis
C) Ancylostoma duodenale
D) Trichuris trichiura

✅ Answer: B) Enterobius vermicularis
Explanation: Pinworm females lay eggs at night → itching.

Q42. Which fungus is most common cause of oral thrush?

A) Aspergillus fumigatus
B) Candida albicans
C) Cryptococcus neoformans
D) Histoplasma capsulatum

✅ Answer: B) Candida albicans
Explanation: Overgrowth of Candida → white plaques in mouth.

Q43. Which fungus causes “valley fever”?

A) Coccidioides immitis
B) Histoplasma capsulatum
C) Blastomyces dermatitidis
D) Aspergillus flavus

✅ Answer: A) Coccidioides immitis
Explanation: Endemic in desert regions of USA (San Joaquin Valley).

Q44. Which fungus produces aflatoxin, a potent hepatocarcinogen?

A) Candida albicans
B) Aspergillus flavus
C) Cryptococcus neoformans
D) Rhizopus stolonifer

✅ Answer: B) Aspergillus flavus
Explanation: Aflatoxins contaminate peanuts, maize, → liver cancer risk.

Q45. Which virus causes infectious mononucleosis?

A) Cytomegalovirus (CMV)
B) Epstein–Barr virus (EBV)
C) Human papillomavirus (HPV)
D) Hepatitis B virus

✅ Answer: B) Epstein–Barr virus
Explanation: EBV causes fever, pharyngitis, lymphadenopathy, atypical lymphocytes.

Q46. Which hepatitis virus is DNA-based?

A) Hepatitis A
B) Hepatitis B
C) Hepatitis C
D) Hepatitis E

✅ Answer: B) Hepatitis B
Explanation: Hepatitis B virus is a partially dsDNA hepadnavirus. Others are RNA viruses.

Q47. Which virus causes rabies?

A) Lyssavirus
B) Paramyxovirus
C) Flavivirus
D) Arenavirus

✅ Answer: A) Lyssavirus
Explanation: Rabies virus is a bullet-shaped RNA virus (family Rhabdoviridae).

Q48. The influenza virus genome consists of:

A) Segmented RNA
B) Linear dsDNA
C) Circular DNA
D) Single RNA segment

✅ Answer: A) Segmented RNA
Explanation: Influenza virus has 8 segmented negative-sense RNA strands, allowing antigenic shift.

Q49. Which virus is oncogenic and associated with cervical cancer?

A) EBV
B) Hepatitis C virus
C) HPV (Human papillomavirus)
D) HTLV-1

✅ Answer: C) HPV
Explanation: HPV types 16 & 18 strongly linked to cervical carcinoma.

Q50. Which diagnostic test detects HIV antibodies?

A) ELISA
B) Gram stain
C) India ink test
D) Ziehl–Neelsen stain

✅ Answer: A) ELISA
Explanation: Initial HIV screening is via ELISA for anti-HIV antibodies, followed by confirmatory Western blot.

Microbiology MCQs – Part 3 (Q51–Q75)

Q51. The causative agent of tuberculosis is:

A) Mycobacterium leprae
B) Mycobacterium tuberculosis
C) Nocardia asteroides
D) Corynebacterium diphtheriae

✅ Answer: B) Mycobacterium tuberculosis
Explanation: Acid-fast bacillus causing pulmonary & extrapulmonary TB.

Q52. The Mantoux test is used for detection of:

A) Active tuberculosis infection
B) Latent or prior exposure to Mycobacterium tuberculosis
C) Leprosy
D) Syphilis

✅ Answer: B) Latent or prior exposure
Explanation: PPD injection causes delayed hypersensitivity in previously sensitized individuals.

Q53. Which mycobacterium causes leprosy?

A) M. kansasii
B) M. tuberculosis
C) M. leprae
D) M. avium

✅ Answer: C) M. leprae
Explanation: Causes Hansen’s disease (leprosy).

Q54. Which stain is used for acid-fast bacilli?

A) Gram stain
B) Ziehl–Neelsen stain
C) India ink
D) Giemsa stain

✅ Answer: B) Ziehl–Neelsen stain
Explanation: Detects acid-fast organisms like Mycobacterium and Nocardia.

Q55. Which bacteria cause syphilis?

A) Treponema pallidum
B) Borrelia burgdorferi
C) Leptospira interrogans
D) Neisseria gonorrhoeae

✅ Answer: A) Treponema pallidum
Explanation: A spirochete causing sexually transmitted syphilis.

Q56. Which organism causes Lyme disease?

A) Treponema pallidum
B) Borrelia burgdorferi
C) Rickettsia prowazekii
D) Leptospira interrogans

✅ Answer: B) Borrelia burgdorferi
Explanation: Tick-borne spirochete causing Lyme disease.

Q57. The characteristic lesion of primary syphilis is:

A) Maculopapular rash
B) Painless chancre
C) Condyloma lata
D) Gummas

✅ Answer: B) Painless chancre
Explanation: Primary stage of syphilis presents with painless genital ulcer.

Q58. Which bacterium causes whooping cough?

A) Bordetella pertussis
B) Haemophilus influenzae
C) Klebsiella pneumoniae
D) Legionella pneumophila

✅ Answer: A) Bordetella pertussis
Explanation: Produces pertussis toxin → paroxysmal coughing.

**Q59. Which medium is used for growing Mycobacterium tuberculosis?**

A) Sabouraud agar
B) Lowenstein–Jensen medium
C) Chocolate agar
D) Blood agar

✅ Answer: B) Lowenstein–Jensen medium
Explanation: Egg-based medium for culturing M. tuberculosis.

Q60. Which bacterium causes gonorrhea?

A) Neisseria meningitidis
B) Neisseria gonorrhoeae
C) Haemophilus ducreyi
D) Chlamydia trachomatis

✅ Answer: B) Neisseria gonorrhoeae
Explanation: Gram-negative diplococcus causing STI gonorrhea.

Q61. Which bacterium causes chancroid (painful ulcer)?

A) Treponema pallidum
B) Neisseria gonorrhoeae
C) Haemophilus ducreyi
D) Chlamydia trachomatis

✅ Answer: C) Haemophilus ducreyi
Explanation: Causes painful genital ulcers, unlike syphilis.

Q62. Which virus causes German measles (rubella)?

A) Paramyxovirus
B) Rubivirus (Togaviridae)
C) Morbillivirus
D) Enterovirus

✅ Answer: B) Rubivirus
Explanation: Rubella virus → mild measles-like illness, teratogenic in pregnancy.

Q63. Which virus causes chickenpox?

A) Variola virus
B) Varicella-zoster virus
C) Measles virus
D) Rubella virus

✅ Answer: B) Varicella-zoster virus
Explanation: VZV causes chickenpox and reactivation causes shingles.

Q64. Which virus causes measles (rubeola)?

A) Paramyxovirus (Morbillivirus)
B) Rubella virus
C) Adenovirus
D) Influenza virus

✅ Answer: A) Paramyxovirus (Morbillivirus)
Explanation: Measles virus causes fever, Koplik spots, rash.

Q65. Which virus causes smallpox?

A) Vaccinia virus
B) Variola virus
C) Cowpox virus
D) Monkeypox virus

✅ Answer: B) Variola virus
Explanation: Variola virus (Poxviridae) → eradicated by vaccination.

Q66. Which virus causes poliomyelitis?

A) Enterovirus (Poliovirus)
B) Rotavirus
C) Norovirus
D) Coxsackievirus

✅ Answer: A) Poliovirus (Enterovirus C)
Explanation: Poliovirus damages motor neurons → paralysis.

Q67. Rotavirus is an important cause of:

A) Respiratory infection in infants
B) Severe diarrhea in children
C) Jaundice in adults
D) Meningitis

✅ Answer: B) Severe diarrhea in children
Explanation: Rotavirus → major cause of pediatric gastroenteritis.

Q68. Norovirus is a leading cause of:

A) Traveler’s diarrhea
B) Foodborne viral gastroenteritis
C) Hepatitis
D) Childhood measles

✅ Answer: B) Foodborne viral gastroenteritis
Explanation: Causes outbreaks of acute vomiting/diarrhea in groups.

Q69. Which hepatitis virus is transmitted mainly by contaminated water (fecal-oral)?

A) Hepatitis A & E
B) Hepatitis B & D
C) Hepatitis C
D) Hepatitis G

✅ Answer: A) Hepatitis A & E
Explanation: Both are RNA viruses, spread by ingestion of contaminated food/water.

Q70. Which virus is strongly associated with hepatocellular carcinoma (HCC)?

A) Hepatitis A
B) Hepatitis B
C) Hepatitis E
D) Adenovirus

✅ Answer: B) Hepatitis B
Explanation: Chronic HBV infection predisposes to HCC.

Q71. Which protozoan causes African sleeping sickness?

A) Leishmania donovani
B) Trypanosoma brucei
C) Plasmodium vivax
D) Giardia lamblia

✅ Answer: B) Trypanosoma brucei
Explanation: Transmitted by tsetse fly.

Q72. Which protozoan causes Chagas disease (American trypanosomiasis)?

A) Trypanosoma cruzi
B) Leishmania braziliensis
C) Plasmodium falciparum
D) Entamoeba histolytica

✅ Answer: A) Trypanosoma cruzi
Explanation: Transmitted by Reduviid bug (kissing bug).

Q73. Which parasite causes toxoplasmosis?

A) Toxoplasma gondii
B) Cryptosporidium parvum
C) Giardia lamblia
D) Trichomonas vaginalis

✅ Answer: A) Toxoplasma gondii
Explanation: Cats are definitive hosts; congenital infection can damage fetus.

Q74. Which parasite causes watery diarrhea in AIDS patients?

A) Cryptosporidium parvum
B) Giardia lamblia
C) Isospora belli
D) Cyclospora cayetanensis

✅ Answer: A) Cryptosporidium parvum
Explanation: Oocysts resist chlorination, cause chronic diarrhea in immunocompromised.

Q75. Which parasite is transmitted by ingestion of undercooked fish and causes vitamin B12 deficiency?

A) Taenia saginata
B) Taenia solium
C) Diphyllobothrium latum
D) Enterobius vermicularis

✅ Answer: C) Diphyllobothrium latum
Explanation: Fish tapeworm absorbs vitamin B12 → megaloblastic anemia.

Microbiology MCQs – Part 4 (Q76–Q100)

Q76. Which bacterium is commonly linked to hospital-acquired pneumonia and has a red “currant jelly” sputum?

A) Streptococcus pneumoniae
B) Klebsiella pneumoniae
C) Haemophilus influenzae
D) Legionella pneumophila

✅ Answer: B) Klebsiella pneumoniae
Explanation: Causes severe pneumonia in alcoholics/diabetics with thick, bloody sputum.

Q77. Which bacterium causes Legionnaires’ disease?

A) Legionella pneumophila
B) Mycoplasma pneumoniae
C) Chlamydophila pneumoniae
D) Streptococcus pneumoniae

✅ Answer: A) Legionella pneumophila
Explanation: Spread via contaminated water aerosols (AC, cooling towers).

Q78. Which organism causes atypical “walking” pneumonia in young adults?

A) Klebsiella pneumoniae
B) Mycoplasma pneumoniae
C) Streptococcus pneumoniae
D) Haemophilus influenzae

✅ Answer: B) Mycoplasma pneumoniae
Explanation: Lacks cell wall, causes mild pneumonia; not visible on Gram stain.

Q79. Which bacterium is the leading cause of community-acquired pneumonia?

A) Streptococcus pneumoniae
B) Klebsiella pneumoniae
C) Mycoplasma pneumoniae
D) Haemophilus influenzae

✅ Answer: A) Streptococcus pneumoniae
Explanation: Gram-positive diplococcus; most common cause of lobar pneumonia.

Q80. Which bacterium causes anthrax?

A) Bacillus cereus
B) Bacillus anthracis
C) Clostridium tetani
D) Clostridium perfringens

✅ Answer: B) Bacillus anthracis
Explanation: Spore-forming bacillus with poly-D-glutamate capsule and anthrax toxin.

Q81. Which bacterium produces tetanus toxin?

A) Clostridium perfringens
B) Clostridium difficile
C) Clostridium tetani
D) Clostridium botulinum

✅ Answer: C) Clostridium tetani
Explanation: Tetanospasmin inhibits inhibitory neurotransmitters → spastic paralysis.

Q82. Which bacterium produces botulinum toxin?

A) Clostridium botulinum
B) Clostridium difficile
C) Clostridium perfringens
D) Clostridium tetani

✅ Answer: A) Clostridium botulinum
Explanation: Blocks acetylcholine release → flaccid paralysis.

Q83. Which bacterium causes pseudomembranous colitis after antibiotic use?

A) Escherichia coli
B) Clostridium difficile
C) Salmonella typhi
D) Shigella flexneri

✅ Answer: B) Clostridium difficile
Explanation: Overgrowth after broad-spectrum antibiotics; toxins A & B damage colon.

Q84. Which bacterium causes typhoid fever?

A) Salmonella Typhi
B) Shigella sonnei
C) Vibrio cholerae
D) Escherichia coli

✅ Answer: A) Salmonella Typhi
Explanation: Transmitted via fecal-oral route; causes step-ladder fever & intestinal perforation.

Q85. Which bacterium causes bacillary dysentery?

A) Salmonella
B) Shigella dysenteriae
C) Vibrio cholerae
D) E. coli

✅ Answer: B) Shigella dysenteriae
Explanation: Invades colonic mucosa → bloody diarrhea with mucus.

Q86. Which bacterium causes cholera?

A) Salmonella enteritidis
B) Shigella sonnei
C) Vibrio cholerae
D) Campylobacter jejuni

✅ Answer: C) Vibrio cholerae
Explanation: Cholera toxin ↑cAMP → profuse watery diarrhea (rice-water stools).

Q87. Which bacterium is a common cause of bacterial gastroenteritis from poultry?

A) Vibrio cholerae
B) Salmonella enteritidis
C) Campylobacter jejuni
D) E. coli

✅ Answer: C) Campylobacter jejuni
Explanation: Leading cause of bacterial diarrhea worldwide, often from undercooked chicken.

Q88. Which organism is linked to hemolytic uremic syndrome (HUS)?

A) Shigella dysenteriae
B) E. coli O157:H7
C) Salmonella typhi
D) Vibrio cholerae

✅ Answer: B) E. coli O157:H7
Explanation: Shiga-like toxin damages kidneys → hemolysis, thrombocytopenia, renal failure.

Q89. Which fungus is most associated with meningitis in AIDS patients?

A) Candida albicans
B) Cryptococcus neoformans
C) Aspergillus fumigatus
D) Histoplasma capsulatum

✅ Answer: B) Cryptococcus neoformans
Explanation: Encapsulated yeast detected by India ink & cryptococcal antigen test.

Q90. Which fungus is endemic in the Ohio and Mississippi River valleys and found in bird droppings?

A) Blastomyces dermatitidis
B) Histoplasma capsulatum
C) Coccidioides immitis
D) Candida albicans

✅ Answer: B) Histoplasma capsulatum
Explanation: Dimorphic fungus associated with bat/bird droppings.

Q91. Which fungus is linked to mold with acute-angle branching hyphae?

A) Candida albicans
B) Cryptococcus neoformans
C) Aspergillus fumigatus
D) Rhizopus

✅ Answer: C) Aspergillus fumigatus
Explanation: Produces acute-angle septate hyphae, can cause aspergillosis.

Q92. Which fungus is associated with diabetic ketoacidosis and produces broad, non-septate hyphae?

A) Aspergillus
B) Rhizopus
C) Candida
D) Histoplasma

✅ Answer: B) Rhizopus
Explanation: Causes mucormycosis in diabetics.

Q93. Which bacterium causes scarlet fever?

A) Streptococcus pyogenes
B) Streptococcus pneumoniae
C) Staphylococcus aureus
D) Corynebacterium diphtheriae

✅ Answer: A) Streptococcus pyogenes
Explanation: Produces erythrogenic toxin → rash, strawberry tongue.

Q94. Which virus is responsible for hand, foot, and mouth disease?

A) Enterovirus (Coxsackie A)
B) Adenovirus
C) Herpes simplex virus
D) Parvovirus B19

✅ Answer: A) Coxsackie A virus
Explanation: Enterovirus causes blisters on mouth, palms, soles.

Q95. Which virus causes erythema infectiosum (slapped-cheek disease)?

A) Parvovirus B19
B) Measles virus
C) Rubella virus
D) Adenovirus

✅ Answer: A) Parvovirus B19
Explanation: Causes fifth disease, especially in children.

Q96. Which virus causes hemorrhagic fever with renal syndrome (HFRS)?

A) Dengue virus
B) Hantavirus
C) Ebola virus
D) Marburg virus

✅ Answer: B) Hantavirus
Explanation: Transmitted by rodent excreta; causes pulmonary & renal syndromes.

Q97. Which arbovirus causes dengue fever?

A) Flavivirus
B) Reovirus
C) Rhabdovirus
D) Paramyxovirus

✅ Answer: A) Flavivirus
Explanation: Dengue virus (Flaviviridae), transmitted by Aedes aegypti.

Q98. Which virus causes hemorrhagic fever with high mortality and filamentous shape?

A) Influenza virus
B) Ebola virus
C) Hantavirus
D) Dengue virus

✅ Answer: B) Ebola virus
Explanation: Ebola (filovirus) causes severe viral hemorrhagic fever.

Q99. Which laboratory technique amplifies DNA?

A) ELISA
B) PCR (Polymerase Chain Reaction)
C) Western blot
D) Electrophoresis

✅ Answer: B) PCR
Explanation: PCR rapidly amplifies specific DNA sequences.

Q100. Western blotting is used to detect:

A) Specific proteins
B) DNA sequences
C) RNA sequences
D) Carbohydrates

✅ Answer: A) Specific proteins
Explanation:

Southern blot → DNA
Northern blot → RNA
Western blot → proteins

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