USMLE Microbiology & Immunology 2 — Questions and Answers
Question 1: A sputum culture grows gram-positive cocci in clusters that are catalase-positive and coagulase-positive. What organism is this?
- Staphylococcus aureus (Correct answer)
- Staphylococcus epidermidis
- Streptococcus pyogenes
- Enterococcus faecalis
Correct answer: Staphylococcus aureus
Coagulase-positive Staphylococcus aureus is distinguished from other staphylococci by its production of coagulase, which converts fibrinogen to fibrin.
Question 2: Which type of T helper cell is responsible for orchestrating the response against intracellular pathogens by activating macrophages through IFN-gamma?
- Th1 (Correct answer)
- Th2
- Th17
- Treg
Correct answer: Th1
Th1 cells produce IFN-gamma and TNF-beta, activating macrophages to destroy intracellular pathogens and driving cell-mediated immunity.
Question 3: A child is vaccinated with a live attenuated vaccine. Why is this vaccine contraindicated in immunocompromised patients?
- The attenuated virus can revert to virulence and cause disease in immunocompromised hosts (Correct answer)
- The vaccine causes excessive antibody production
- The killed virus cannot stimulate immunity
- Adjuvants in live vaccines cause systemic inflammation
Correct answer: The attenuated virus can revert to virulence and cause disease in immunocompromised hosts
Live attenuated vaccines can cause disease in immunocompromised patients because the attenuated organism may revert to virulence when the immune system cannot control even weakened pathogens.
Question 4: A CSF sample shows gram-negative diplococci. What is the most likely causative organism in an 18-year-old college student?
- Neisseria meningitidis (Correct answer)
- Haemophilus influenzae
- Streptococcus pneumoniae
- Listeria monocytogenes
Correct answer: Neisseria meningitidis
Neisseria meningitidis is the most common cause of bacterial meningitis in adolescents and young adults, particularly in close-contact settings like college dormitories.
Question 5: Which mechanism allows bacteria to acquire antibiotic resistance from the environment by taking up naked DNA?
- Transformation (Correct answer)
- Conjugation
- Transduction
- Vertical transmission
Correct answer: Transformation
Transformation is the uptake of naked DNA from the environment by naturally competent bacteria, allowing acquisition of resistance genes from dead bacteria.
Question 6: A positive tuberculin skin test (PPD) indicates which type of hypersensitivity reaction?
- Type IV (delayed-type hypersensitivity, T-cell mediated) (Correct answer)
- Type I (IgE-mediated)
- Type II (antibody-mediated cytotoxicity)
- Type III (immune complex)
Correct answer: Type IV (delayed-type hypersensitivity, T-cell mediated)
PPD reaction is a Type IV delayed-type hypersensitivity reaction mediated by sensitized memory T cells that recognize tuberculin antigens and recruit macrophages, peaking at 48-72 hours.
USMLE Microbiology & Immunology 2
A sputum culture grows gram-positive cocci in clusters that are catalase-positive and coagulase-positive.
What organism is this?