MBBS Microbiology 2 — Questions and Answers
Question 1: Which fungal infection is most commonly seen in immunocompromised patients and presents with a 'soap bubble' appearance on brain imaging?
- Aspergillosis
- Cryptococcosis (Correct answer)
- Candidiasis
- Mucormycosis
Correct answer: Cryptococcosis
Cryptococcus neoformans causes cryptococcal meningitis in immunocompromised patients; the mucoid capsule produces the 'soap bubble' lesions seen on CT/MRI of the brain.
Question 2: A patient presents with rice-water stools and severe dehydration. The causative organism produces a toxin that permanently activates Gs protein. Which organism is responsible?
- Salmonella typhi
- Shigella dysenteriae
- Vibrio cholerae (Correct answer)
- Campylobacter jejuni
Correct answer: Vibrio cholerae
Vibrio cholerae produces cholera toxin that permanently ADP-ribosylates Gs alpha subunit, leading to constitutive activation of adenylyl cyclase, massive chloride secretion, and rice-water diarrhea.
Question 3: Which antibiotic acts by inhibiting bacterial cell wall synthesis by binding to the D-Ala-D-Ala terminus of peptidoglycan precursors?
- Penicillin
- Vancomycin (Correct answer)
- Ciprofloxacin
- Rifampin
Correct answer: Vancomycin
Vancomycin binds to the D-Ala-D-Ala terminus of peptidoglycan precursors, preventing transglycosylation and transpeptidation, thereby inhibiting cell wall synthesis.
Question 4: A 3-year-old presents with a 'barking cough' and inspiratory stridor. Which virus is the most common cause of this condition (croup)?
- Respiratory syncytial virus
- Parainfluenza virus (Correct answer)
- Influenza A virus
- Adenovirus
Correct answer: Parainfluenza virus
Parainfluenza virus type 1 is the most common cause of croup (laryngotracheobronchitis), characterized by subglottic edema producing the classic barking cough and inspiratory stridor.
Question 5: Which organism causes 'walking pneumonia' and lacks a cell wall, making it intrinsically resistant to beta-lactam antibiotics?
- Legionella pneumophila
- Mycoplasma pneumoniae (Correct answer)
- Chlamydia pneumoniae
- Bordetella pertussis
Correct answer: Mycoplasma pneumoniae
Mycoplasma pneumoniae lacks a cell wall entirely, making it resistant to all beta-lactam antibiotics; it causes atypical 'walking pneumonia' with mild symptoms and a dry cough.
Question 6: Which malaria parasite is associated with tertian fever occurring every 48 hours and can remain dormant in the liver as hypnozoites?
- Plasmodium falciparum
- Plasmodium vivax (Correct answer)
- Plasmodium malariae
- Plasmodium knowlesi
Correct answer: Plasmodium vivax
Plasmodium vivax causes benign tertian malaria with 48-hour fever cycles and forms dormant hypnozoites in the liver, responsible for relapses months to years after initial infection.
Question 7: In the complement system, which component acts as the convergence point for both the classical and alternative pathways?
- C1q
- C3 (Correct answer)
- C5
- C9
Correct answer: C3
C3 is the central convergence point; both the classical pathway (via C4b2a C3 convertase) and the alternative pathway (via C3bBb C3 convertase) cleave C3 into C3a and C3b.
MBBS Microbiology 2
Which fungal infection is most commonly seen in immunocompromised patients and presents with a 'soap bubble' appearance on brain imaging?