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Question 1. Which component of the chain of infection refers to the place where a pathogen normally lives and multiplies? A) Portal of entry B) Reservoir C) Mode of transmission D) Susceptible host Answer: B Explanation: A reservoir is the environment in which an infectious agent thrives and multiplies, such as water, soil, or a human carrier. Question 2. The most common mode of transmission for Streptococcus pyogenes causing strep throat is: A) Airborne droplets B) Direct skin contact C) Vector bite D) Fecal‑oral route Answer: A Explanation: S. pyogenes spreads primarily via respiratory droplets expelled when an infected person coughs or sneezes. Question 3. Which of the following best describes the innate immune response? A) Production of specific antibodies after exposure B) Immediate, non‑specific defense mechanisms C) Memory cells that recognize previous pathogens D) Activation of cytotoxic T‑cells only Answer: B Explanation: Innate immunity provides rapid, non‑specific protection (e.g., skin, phagocytes, complement) without prior sensitization.
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Question 4. In Gram staining, a bacterium that appears pink under the microscope is most likely:** A) Gram‑positive B) Gram‑negative C) Acid‑fast D) Endospore‑forming Answer: B Explanation: Pink (or red) coloration indicates the organism did not retain crystal violet and is therefore Gram‑negative. Question 5. Which of the following organisms is an obligate anaerobe? A) Staphylococcus aureus B) Pseudomonas aeruginosa C) Clostridium difficile D) Neisseria gonorrhoeae Answer: C Explanation: C. difficile grows only in the absence of oxygen, classifying it as an obligate anaerobe. Question 6. The incubation period of a viral infection refers to: A) Time from symptom onset to recovery B) Time from exposure to first detectable symptoms C) Time during which the virus is no longer contagious D) Time needed for vaccine-induced immunity to develop Answer: B Explanation: Incubation is the interval between pathogen exposure and the appearance of initial clinical signs. Question 7. Which antimicrobial class is most associated with tendon rupture as a black‑box warning?
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B) Histamine release due to rapid infusion C) Bacterial endotoxin contamination D) Allergic IgE‑mediated reaction Answer: B Explanation: Rapid vancomycin infusion triggers non‑IgE histamine release, producing flushing and pruritus. Question 11. Which hepatitis virus is transmitted primarily via the fecal‑oral route? A) Hepatitis A B) Hepatitis B C) Hepatitis C D) Hepatitis D Answer: A Explanation: Hepatitis A spreads through ingestion of contaminated food or water, typical of fecal‑oral transmission. Question 12. The most appropriate isolation precaution for a patient with airborne tuberculosis is: A) Contact precautions with gown and gloves B) Droplet precautions with surgical mask C) Airborne precautions with N95 respirator D) Standard precautions only Answer: C Explanation: Airborne TB requires a fit‑tested N95 respirator to filter inhaled Mycobacterium tuberculosis particles. Question 13. A positive acid‑fast stain is characteristic of which organism? A) Streptococcus pyogenes B) Mycobacterium tuberculosis
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C) Neisseria meningitidis D) Clostridium perfringens Answer: B Explanation: Mycobacteria possess a lipid‑rich cell wall that retains carbol‑fuchsin after acid decolorization, giving a positive acid‑fast result. Question 14. Which of the following best describes the “prodromal” stage of infection? A) Period of complete recovery B) Phase where nonspecific symptoms appear before full disease C) Time when the pathogen is no longer viable D) Stage characterized by severe organ damage Answer: B Explanation: The prodromal stage involves vague, early symptoms (e.g., malaise, low‑grade fever) that precede the full illness. Question 15. Which antiviral drug class is used as first‑line therapy for chronic hepatitis C infection? A) Nucleoside reverse transcriptase inhibitors B) Protease inhibitors (e.g., sofosbuvir) C) Neuraminidase inhibitors D) Integrase strand transfer inhibitors Answer: B Explanation: Direct‑acting antivirals such as sofosbuvir inhibit HCV NS5B polymerase and are first‑line for chronic hepatitis C. Question 16. An immunocompromised patient develops a painless ulcer on the genitalia. Which organism is most likely responsible? A) Treponema pallidum B) Herpes simplex virus C) Chlamydia trachomatis
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Answer: D Explanation: qSOFA uses mental status, systolic BP, and respiratory rate; lactate is part of the full SOFA, not qSOFA. Question 20. Which bacterial toxin is responsible for the “rice‑water” stools of cholera? A) Shiga toxin B) Cholera toxin (enterotoxin) C) Exotoxin A D) Botulinum toxin Answer: B Explanation: Cholera toxin ADP‑ribosylates G‑protein, increasing cAMP and causing profuse watery diarrhea. Question 21. A patient with a recent travel history to sub‑Saharan Africa presents with fever, chills, and hemolysis. Which parasite is most likely? A) Plasmodium falciparum B) Giardia lamblia C) Entamoeba histolytica D) Trypanosoma cruzi Answer: A Explanation: P. falciparum causes severe malaria with hemolysis and is prevalent in sub‑Saharan Africa. Question 22. Which of the following antibiotics is contraindicated in patients with a known severe penicillin allergy due to high cross‑reactivity? A) Azithromycin B) Ceftriaxone C. Clindamycin D) Doxycycline
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Answer: B Explanation: Cephalosporins share a β‑lactam ring with penicillins; severe penicillin allergy raises risk of cross‑reaction, especially with first‑generation agents. Question 23. The most appropriate vaccination for a 30‑year‑old adult without prior immunization against hepatitis B is: A) Single‑dose inactivated vaccine B) Two‑dose recombinant vaccine given 0 and 1 month C) Three‑dose series at 0, 1, and 6 months D) Live‑attenuated vaccine at 0 months only Answer: C Explanation: The standard hepatitis B vaccine schedule is a three‑dose series (0, 1, 6 months) for adults. Question 24. Which of the following best describes the mechanism of resistance for methicillin‑resistant Staphylococcus aureus (MRSA)? A) Production of β‑lactamase that hydrolyzes penicillins B) Altered penicillin‑binding protein (PBP2a) with low affinity for β‑lactams C) Efflux pump that removes macrolides D. Methylation of 23S rRNA Answer: B Explanation: MRSA carries the mecA gene encoding PBP2a, which has reduced affinity for β‑lactam antibiotics. Question 25. Which of the following is a classic finding in the cerebrospinal fluid of patients with viral (aseptic) meningitis? A) Elevated neutrophils > 80% B) Low glucose < 30 mg/dL C) Predominant lymphocytosis D) Positive Gram stain
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Explanation: A macrolide such as azithromycin covers typical and atypical pathogens for outpatient CAP in healthy adults. Question 29. Which laboratory finding is most characteristic of severe sepsis? A) Serum albumin > 4 g/dL B) Lactate > 2 mmol/L C) Hemoglobin > 15 g/dL D) Platelet count > 300,000/μL Answer: B Explanation: Elevated lactate reflects tissue hypoperfusion and is a key marker of severe sepsis. Question 30. Which of the following fungi produces a distinctive “moldy” odor on culture and forms septate hyphae with conidia? A) Candida albicans B) Aspergillus fumigatus C) Cryptococcus neoformans D) Histoplasma capsulatum Answer: B Explanation: A. fumigatus grows rapidly with a characteristic earthy odor and produces septate hyphae with conidial heads. Question 31. In HIV infection, which CD4+ count range defines AIDS‑defining immunosuppression? A) > 500 cells/μL B) 350‑ 500 cells/μL C) 200‑ 350 cells/μL D) < 200 cells/μL Answer: D Explanation: A CD4+ count below 200 cells/μL meets the immunologic criterion for AIDS.
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Question 32. The most common cause of bacterial meningitis in neonates is: A) Streptococcus pneumoniae B) Neisseria meningitidis C) Group B Streptococcus (Streptococcus agalactiae) D) Listeria monocytogenes Answer: C Explanation: Group B Streptococcus is the leading pathogen causing early‑onset neonatal meningitis. Question 33. Which of the following is a characteristic side effect of amphotericin B that requires pre‑hydration? A) Hepatotoxicity B. Nephrotoxicity C) Bone marrow suppression D) Hypoglycemia Answer: B Explanation: Amphotericin B can cause dose‑dependent renal tubular injury; aggressive hydration reduces nephrotoxicity. Question 34. Which of the following vaccines is contraindicated in a patient with severe immunodeficiency? A. Inactivated influenza vaccine B) Live‑attenuated varicella vaccine C) Pneumococcal conjugate vaccine (PCV13) D) Hepatitis B vaccine Answer: B Explanation: Live‑attenuated vaccines (e.g., varicella) can cause disease in severely immunocompromised individuals.
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Question 38. Which of the following organisms is a common cause of catheter‑related bloodstream infection in intensive care units? A) Streptococcus agalactiae B) Pseudomonas aeruginosa C) Enterococcus faecalis D) Candida glabrata Answer: B Explanation: P. aeruginosa thrives in moist environments and frequently contaminates intravascular catheters in ICU settings. Question 39. In the context of infection control, the “5 Moments for Hand Hygiene” include all EXCEPT: A) Before patient contact B) Before aseptic task C) After removing gloves D) After patient discharge from the unit Answer: D Explanation: The five moments are before patient contact, before aseptic task, after body fluid exposure risk, after patient contact, and after removing gloves; discharge is not a defined moment. Question 40. Which of the following is the most sensitive test for diagnosing active tuberculosis? A) Chest X‑ray alone B) Sputum acid‑fast smear microscopy C) Nucleic acid amplification test (NAAT) on sputum D) Tuberculin skin test (TST) Answer: C Explanation: NAAT (e.g., GeneXpert) detects MTB DNA rapidly and with higher sensitivity than smear microscopy.
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Question 41. A 45‑year‑old man with a prosthetic heart valve presents with fever and a new murmur. Blood cultures grow gram‑positive cocci in clusters, catalase‑positive, coagulase‑negative. Which organism is most likely? A) Staphylococcus aureus B. Staphylococcus epidermidis C) Enterococcus faecalis D) Streptococcus viridans Answer: B Explanation: S. epidermidis is coagulase‑negative, catalase‑positive, and a common cause of prosthetic‑device infections. Question 42. Which antiviral medication is a nucleoside analog that inhibits reverse transcriptase and is a cornerstone of HIV antiretroviral therapy? A) Oseltamivir B) Zidovudine (AZT) C) Acyclovir D) Sofosbuvir Answer: B Explanation: Zidovudine is a thymidine analog that blocks HIV reverse transcriptase, forming part of combination ART. Question 43. The most common cause of healthcare‑associated pneumonia (HAP) is: A) Streptococcus pneumoniae B) Pseudomonas aeruginosa C) Mycoplasma pneumoniae D) Legionella pneumophila Answer: B Explanation: P. aeruginosa is a frequent Gram‑negative pathogen in HAP, especially in mechanically ventilated patients.
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Question 47. Which of the following best describes the mechanism of action of β‑lactam antibiotics? A) Inhibit protein synthesis by binding the 30S ribosomal subunit B) Disrupt cell membrane integrity C. Inhibit bacterial cell‑wall synthesis by binding penicillin‑binding proteins D) Inhibit nucleic acid synthesis by targeting DNA gyrase Answer: C Explanation: β‑lactams (penicillins, cephalosporins) block transpeptidation of peptidoglycan, leading to cell‑wall failure. Question 48. The most appropriate method to prevent transmission of Clostridioides difficile spores in a hospital ward is: A) Airborne isolation with N95 respirator B) Contact precautions with gown, gloves, and dedicated equipment C) Droplet precautions with surgical mask D) No special precautions; routine cleaning suffices Answer: B Explanation: C. difficile spreads via fecal‑oral route; contact precautions and thorough environmental cleaning are essential. Question 49. Which of the following is a hallmark laboratory finding in a patient with severe COVID‑19 infection? A) Lymphocytosis B) Elevated D‑dimer levels C) Low C‑reactive protein (CRP) D) Decreased ferritin Answer: B Explanation: Elevated D‑dimer reflects hypercoagulability and is associated with severe COVID‑ 19 disease.
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Question 50. Which of the following organisms is a common cause of fungal sinusitis in immunocompromised patients? A) Candida albicans B) Aspergillus fumigatus C) Trichophyton mentagrophytes D) Cryptococcus neoformans Answer: B Explanation: Invasive sinusitis in neutropenic patients is frequently due to Aspergillus species. Question 51. The most appropriate treatment for uncomplicated Helicobacter pylori infection includes: A) Metronidazole alone for 7 days B. Triple therapy with a PPI, clarithromycin, and amoxicillin for 14 days C) Vancomycin monotherapy D) Ciprofloxacin for 3 days Answer: B Explanation: Standard first‑line eradication uses a proton‑pump inhibitor plus clarithromycin and amoxicillin (or metronidazole) for 14 days. Question 52. Which of the following best explains why Neisseria gonorrhoeae has developed resistance to penicillin? A. Production of a β‑lactamase enzyme B) Altered ribosomal binding site C) Increased efflux pump activity for macrolides D) Mutation of DNA gyrase Answer: A Explanation: Penicillin‑producing N. gonorrhoeae strains acquire plasmid‑mediated β‑lactamase, hydrolyzing the drug.
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Question 56. Which of the following is the most effective strategy to prevent the spread of multidrug‑resistant organisms in a hospital? A) Routine use of broad‑spectrum antibiotics for all patients B) Implementation of antimicrobial stewardship programs C. Discontinuation of hand hygiene protocols D) Isolation of only symptomatic patients Answer: B Explanation: Stewardship programs optimize antibiotic use, reducing selection pressure and the emergence of resistant strains. Question 57. Which of the following best describes the mechanism of action of the antiviral drug oseltamivir? A) Inhibits viral neuraminidase, preventing release of progeny virions B) Blocks viral DNA polymerase C) Interferes with viral entry by binding hemagglutinin D. Inhibits reverse transcriptase Answer: A Explanation: Oseltamivir is a neuraminidase inhibitor that impairs influenza virus budding and release. Question 58. A patient receiving high‑dose intravenous vancomycin develops hypotension, flushing, and erythema of the upper body during infusion. What is the most likely cause? A) Anaphylactic shock B) Red man syndrome due to rapid infusion C) Septic shock from infection D. Renal failure leading to fluid overload Answer: B Explanation: Red man syndrome is a histamine‑mediated reaction caused by rapid vancomycin infusion; slowing the rate mitigates symptoms.
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Question 59. Which of the following organisms is a common cause of otitis externa (“swimmer’s ear”)? A) Streptococcus pneumoniae B) Pseudomonas aeruginosa C) Klebsiella pneumoniae D) Staphylococcus epidermidis Answer: B Explanation: P. aeruginosa thrives in moist environments and frequently infects the external auditory canal after water exposure. Question 60. In a patient with suspected meningococcal meningitis, which prophylactic antibiotic is recommended for close contacts? A) Ciprofloxacin single dose B. Azithromycin for 5 days C) Amoxicillin for 10 days D) Vancomycin IV for 7 days Answer: A Explanation: A single oral dose of ciprofloxacin is recommended for chemoprophylaxis of close contacts of N. meningitidis cases. Question 61. Which of the following is the most common cause of bacterial keratitis in contact lens wearers? A) Streptococcus pneumoniae B) Pseudomonas aeruginosa C) Haemophilus influenzae D) Staphylococcus aureus Answer: B
