MBBS Microbiology 1 — Questions and Answers
Question 1: Which organism is the most common cause of community-acquired pneumonia in adults?
- Haemophilus influenzae
- Streptococcus pneumoniae (Correct answer)
- Mycoplasma pneumoniae
- Klebsiella pneumoniae
Correct answer: Streptococcus pneumoniae
Streptococcus pneumoniae is the leading cause of community-acquired pneumonia in adults, accounting for approximately 30-70% of cases.
Question 2: Which virulence factor of Staphylococcus aureus allows it to resist phagocytosis by inhibiting opsonization?
- Leukocidin
- Protein A (Correct answer)
- Coagulase
- Hyaluronidase
Correct answer: Protein A
Protein A binds to the Fc region of IgG antibodies, preventing opsonization and thereby inhibiting phagocytosis by neutrophils.
Question 3: The Ziehl-Neelsen stain is used to identify organisms that are acid-fast. Which property makes mycobacteria acid-fast?
- High peptidoglycan content
- Presence of mycolic acids in the cell wall (Correct answer)
- Presence of teichoic acids
- Thick polysaccharide capsule
Correct answer: Presence of mycolic acids in the cell wall
Mycolic acids are long-chain fatty acids in the mycobacterial cell wall that retain the carbol fuchsin dye even after acid-alcohol decolorization.
Question 4: Which hepatitis virus is transmitted via the fecal-oral route and does NOT cause chronic infection?
- Hepatitis B
- Hepatitis C
- Hepatitis A (Correct answer)
- Hepatitis D
Correct answer: Hepatitis A
Hepatitis A virus is transmitted fecal-orally and causes only acute self-limiting infection, never leading to chronic hepatitis or carrier state.
Question 5: Clostridium tetani produces tetanospasmin, which causes spastic paralysis. What is its mechanism of action?
- Blocks acetylcholine release at the neuromuscular junction
- Inhibits release of inhibitory neurotransmitters glycine and GABA (Correct answer)
- Activates adenylyl cyclase in neurons
- Cleaves SNARE proteins at peripheral synapses
Correct answer: Inhibits release of inhibitory neurotransmitters glycine and GABA
Tetanospasmin cleaves synaptobrevin (a SNARE protein) in inhibitory interneurons, blocking release of glycine and GABA, resulting in unopposed motor neuron excitation and spastic paralysis.
Question 6: Which test is used to distinguish Streptococcus pyogenes (Group A Strep) from other beta-hemolytic streptococci?
- Optochin sensitivity
- Bacitracin sensitivity (Correct answer)
- CAMP test
- Bile solubility
Correct answer: Bacitracin sensitivity
Streptococcus pyogenes (Group A Strep) is sensitive to bacitracin, which differentiates it from Group B and other beta-hemolytic streptococci.
Question 7: Which type of hypersensitivity reaction is the mechanism behind tuberculin skin test (Mantoux test) positivity?
- Type I (IgE-mediated)
- Type II (cytotoxic)
- Type III (immune complex)
- Type IV (delayed-type/cell-mediated) (Correct answer)
Correct answer: Type IV (delayed-type/cell-mediated)
The Mantoux reaction is a classic Type IV (delayed-type) hypersensitivity, mediated by sensitized T lymphocytes that react to tuberculin antigen 48-72 hours after injection.
MBBS Microbiology 1
Which organism is the most common cause of community-acquired pneumonia in adults?