Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 641: Which is the most important drug for the treatment of persistent tuberculosis?

A. Rifampicin (Correct Answer)
B. Pyrazinamide
C. Isoniazid
D. Ethambutol

Explanation: **Explanation:** The treatment of Tuberculosis (TB) relies on targeting different populations of *Mycobacterium tuberculosis*. The correct answer is **Rifampicin** because of its exceptional **sterilizing activity**. 1. **Why Rifampicin is correct:** *Mycobacterium tuberculosis* exists in four metabolic pools: rapidly growing, intermittently growing (spurters), slowly growing (acidic medium), and **dormant/persistent** bacilli. Rifampicin is the most effective drug against "spurters"—bacilli that undergo short bursts of metabolic activity. By killing these persistent organisms, Rifampicin prevents late relapses and is considered the most important drug for sterilizing the lesion. 2. **Why other options are incorrect:** * **Pyrazinamide:** It is highly effective against bacilli in an acidic medium (intracellular) and is known for its sterilizing activity in the *initial* phase of therapy, but Rifampicin remains the overall most important drug for persistence throughout the course. * **Isoniazid (INH):** It has the highest **Early Bactericidal Activity (EBA)**. It is most effective against rapidly dividing extracellular bacilli, making the patient non-infectious quickly, but it is less effective against dormant/persistent forms. * **Ethambutol:** It is primarily bacteriostatic and is used to prevent the emergence of resistance to other drugs; it has no significant role in killing persistent bacilli. **High-Yield Clinical Pearls for NEET-PG:** * **Highest EBA:** Isoniazid (kills 90% of bacilli in the first 48 hours). * **Best Sterilizing Activity:** Rifampicin (followed by Pyrazinamide). * **Mechanism of Rifampicin:** Inhibits DNA-dependent RNA polymerase. * **Red-orange discoloration** of secretions (urine, sweat, tears) is a harmless side effect of Rifampicin.

Question 642: Which of the following third-generation cephalosporins has activity against MRSA (Methicillin-Resistant Staphylococcus Aureus)?

A. Ceftriaxone
B. Ceftobiprole (Correct Answer)
C. Aztreonam
D. Cephalexin

Explanation: **Explanation:** The correct answer is **Ceftobiprole**. Traditionally, cephalosporins are ineffective against **MRSA** because the bacteria possess an altered penicillin-binding protein (**PBP2a**), which has a low affinity for most beta-lactam antibiotics. However, newer "advanced-generation" cephalosporins (often classified as 5th generation, though sometimes grouped with 3rd/4th in older classifications) like **Ceftobiprole** and **Ceftaroline** have been specifically engineered to bind with high affinity to PBP2a, making them effective against MRSA. **Analysis of Options:** * **Ceftriaxone (Option A):** A classic 3rd generation cephalosporin. While it has excellent Gram-negative coverage and crosses the blood-brain barrier, it lacks activity against MRSA. * **Aztreonam (Option C):** This is a **Monobactam**. It is unique because it only acts against Gram-negative aerobic bacteria (including *Pseudomonas*) and has no activity against Gram-positive organisms like MRSA. * **Cephalexin (Option D):** A 1st generation cephalosporin used primarily for skin infections caused by MSSA (Methicillin-Sensitive *S. aureus*), but it is completely ineffective against MRSA. **High-Yield Clinical Pearls for NEET-PG:** * **Anti-MRSA Cephalosporins:** Remember the "Cepta" duo: **Ceftaroline** and **Ceftobiprole**. * **Anti-Pseudomonal Cephalosporins:** Ceftazidime (3rd gen) and Cefepime (4th gen). * **Ceftriaxone Warning:** It is excreted via bile; therefore, it is the cephalosporin of choice in patients with renal failure, but it can cause "biliary sludge" or pseudolithiasis. * **MRSA Treatment Gold Standard:** Vancomycin remains the traditional drug of choice, but Ceftaroline/Ceftobiprole are vital alternatives for skin and soft tissue infections.

Question 643: Which cephalosporin can cause a disulfiram-like reaction?

A. Cefuroxime
B. Cefamendole (Correct Answer)
C. Ceftazidine
D. Ceftizoxim

Explanation: **Explanation:** The correct answer is **Cefamandole**. This reaction occurs due to the presence of a specific chemical side chain called the **Methylthiotetrazole (MTT) group**. **1. Why Cefamandole is correct:** Certain cephalosporins contain an MTT side chain at the 3-position of the cephalosporin nucleus. This group inhibits the enzyme **aldehyde dehydrogenase**, leading to the accumulation of acetaldehyde in the blood if alcohol is consumed. This results in a "disulfiram-like reaction" characterized by flushing, tachycardia, nausea, vomiting, and hypotension. Additionally, the MTT group can interfere with Vitamin K metabolism, potentially leading to hypoprothrombinemia and bleeding tendencies. **2. Why other options are incorrect:** * **Cefuroxime (Option A):** A second-generation cephalosporin that does not possess the MTT side chain; therefore, it does not cause disulfiram-like reactions. * **Ceftazidime (Option C):** A third-generation cephalosporin primarily used for *Pseudomonas*. It lacks the MTT group. * **Ceftizoxime (Option D):** Another third-generation cephalosporin that does not contain the MTT side chain and is not associated with this reaction. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for MTT-containing Cephalosporins:** "MAN, PERA, TETRA" (**Cefamandole**, **Cefoperazone**, **Cefotetan**). * **Other drugs causing Disulfiram-like reactions:** Metronidazole (most common), Tinidazole, Griseofulvin, and certain Sulfonylureas (Chlorpropamide). * **Clinical Advice:** Patients on these medications must be strictly advised to avoid alcohol during treatment and for up to 72 hours after the last dose.

Question 644: What is the drug of choice for lymphogranuloma venerum?

A. Tetracycline (Correct Answer)
B. Chloramphenicol
C. Erythromycin
D. Ampicillin

Explanation: **Explanation:** **Lymphogranuloma Venereum (LGV)** is a sexually transmitted infection caused by the **L1, L2, and L3 serovars of *Chlamydia trachomatis***. It typically presents with a transient primary genital lesion followed by painful regional lymphadenopathy (the "bubo" formation). 1. **Why Tetracycline is correct:** Tetracyclines, specifically **Doxycycline** (100 mg twice daily for 21 days), are the established **drugs of choice** for LGV. They are highly effective because they inhibit bacterial protein synthesis by binding to the 30S ribosomal subunit, providing excellent intracellular penetration required to eliminate *Chlamydia*. 2. **Why the other options are incorrect:** * **Chloramphenicol:** While it has a broad spectrum, it is not used for LGV due to its potential for serious bone marrow toxicity (aplastic anemia). * **Erythromycin:** This is considered a **second-line alternative** (used primarily in pregnant or lactating women where tetracyclines are contraindicated), but it is not the primary drug of choice. * **Ampicillin:** This cell-wall synthesis inhibitor is ineffective against *Chlamydia* because the organism is an obligate intracellular pathogen with a unique cell wall structure that lacks traditional peptidoglycan. **High-Yield Clinical Pearls for NEET-PG:** * **Groove Sign:** The "Groove sign of Greenblatt" is a classic clinical finding in LGV where the inguinal ligament divides matted lymph nodes. * **Alternative for LGV:** If Doxycycline is unavailable, **Erythromycin base** is the preferred alternative. * **Standard Chlamydia (Non-LGV):** For simple urethritis/cervicitis (Serovars D-K), a single dose of **Azithromycin** (1g) is often preferred for compliance, though Doxycycline remains an option. * **Donovanosis vs. LGV:** Do not confuse LGV with Granuloma Inguinale (Donovanosis), which is caused by *Klebsiella granulomatis* and presents with painless beefy-red ulcers.

Question 645: Which of the following statements about Mebendazole is incorrect?

A. It is safe in pregnancy. (Correct Answer)
B. It is a broad-spectrum antihelminthic.
C. It has relatively low systemic bioavailability.
D. It is active against both larva and adult worms.

Explanation: ### Explanation **Mebendazole** is a synthetic benzimidazole derivative used extensively in the treatment of helminthic infestations. **1. Why Option A is the Correct Answer (Incorrect Statement):** Mebendazole is **not considered safe in pregnancy** (Category C). It has demonstrated **teratogenic and embryotoxic potential** in animal studies (causing skeletal abnormalities and soft tissue defects). Therefore, it is generally contraindicated during pregnancy, especially in the first trimester. For pregnant women with symptomatic intestinal helminthiasis, Pyrantel pamoate is often considered a safer alternative, or treatment is deferred until after delivery. **2. Analysis of Other Options:** * **Option B (Broad-spectrum):** Mebendazole is highly effective against a wide range of nematodes, including *Ascaris lumbricoides* (roundworm), *Enterobius vermicularis* (pinworm), *Trichuris trichiura* (whipworm), and Hookworms (*Ancylostoma* and *Necator*). * **Option C (Low Bioavailability):** It is poorly absorbed from the GI tract (less than 10% systemic absorption). This is actually a clinical advantage for treating intestinal parasites, as it ensures high drug concentrations remain in the gut lumen while minimizing systemic toxicity. * **Option D (Larval and Adult Activity):** Mebendazole is versatile; it kills adult worms and is also effective against their eggs (ovicidal) and larval stages (larvicidal) in many species. **3. Clinical Pearls for NEET-PG:** * **Mechanism of Action:** It binds to helminthic **β-tubulin**, inhibiting microtubule polymerization. This leads to impaired glucose uptake and depletion of glycogen stores in the parasite. * **Administration:** Absorption is significantly **increased when taken with a fatty meal**. * **Drug of Choice:** It is a primary agent for "mixed infestations" due to its broad spectrum. * **Side Effects:** Generally well-tolerated due to low absorption, but high doses (used in hydatid disease) can cause granulocytopenia and alopecia.

Question 646: Which of the following antimicrobial agents can be safely administered to a patient with renal failure?

A. Tetracycline
B. Gentamicin
C. Amphotericin B
D. Doxycycline (Correct Answer)

Explanation: ### Explanation The correct answer is **Doxycycline**. **Why Doxycycline is the correct choice:** Most tetracyclines are primarily excreted by the kidneys and can accumulate in renal failure, leading to increased urea levels (anti-anabolic effect). However, **Doxycycline** is a notable exception. It is primarily excreted through the **bile into the feces** (enterohepatic circulation is not significant for its clearance). Because it does not rely on renal excretion, its half-life remains unchanged in renal impairment, making it the safest tetracycline for patients with kidney disease. **Analysis of Incorrect Options:** * **Tetracycline (Option A):** This is the prototype drug of the class and is primarily excreted via glomerular filtration. It is contraindicated in renal failure as it can cause significant azotemia. * **Gentamicin (Option B):** As an aminoglycoside, it is highly nephrotoxic and eliminated almost entirely by the kidneys. In renal failure, it requires strict dose adjustment and Therapeutic Drug Monitoring (TDM) to prevent further toxicity. * **Amphotericin B (Option C):** This antifungal is notoriously nephrotoxic (causing "shake and bake" reactions and renal tubular acidosis). It is generally avoided or used with extreme caution (liposomal forms) in patients with pre-existing renal compromise. **NEET-PG High-Yield Pearls:** * **"Safe in Renal Failure" Mnemonics:** Remember **"D-M-C"** (Doxycycline, Minocycline, Ceftriaxone) and **"M-E-L-T"** (Moxifloxacin, Erythromycin, Linezolid, Tigecycline) as drugs that generally do not require dose adjustment in renal failure. * **Tetracycline of choice in Renal Failure:** Doxycycline. * **Tetracycline of choice for SIADH:** Demeclocycline (due to its ADH-antagonist property). * **Anti-anabolic effect:** Tetracyclines (except Doxycycline) inhibit protein synthesis, leading to increased amino acid metabolism and a rise in Blood Urea Nitrogen (BUN).

Question 647: Which of the following statements about penicillins is NOT true?

A. Penicillin V is absorbed orally.
B. Benzathine penicillin is a short-acting penicillin. (Correct Answer)
C. Cloxacillin is resistant to β-lactamase and acid.
D. Ampicillin is not resistant to β-lactamases.

Explanation: **Explanation:** The correct answer is **B** because **Benzathine penicillin is a long-acting (repository) penicillin**, not short-acting. It is administered intramuscularly and releases penicillin slowly into the bloodstream, maintaining therapeutic levels for 2–4 weeks. It is the drug of choice for rheumatic fever prophylaxis and syphilis. **Analysis of Options:** * **Option A (True):** Penicillin V (Phenoxymethylpenicillin) is acid-stable, allowing it to survive gastric acid and be **absorbed orally**, unlike Penicillin G. * **Option C (True):** Cloxacillin belongs to the penicillinase-resistant class. It is both **acid-resistant** (can be given orally) and **$\beta$-lactamase resistant**, making it effective against *Staphylococcus aureus*. * **Option D (True):** Ampicillin is an extended-spectrum penicillin. While it covers more Gram-negative bacteria, it is **highly susceptible to $\beta$-lactamases**; hence, it is often combined with inhibitors like sulbactam. **High-Yield Clinical Pearls for NEET-PG:** * **Repository Penicillins:** Procaine Penicillin (24 hours) < Benzathine Penicillin (3–4 weeks). Never give these intravenously (risk of embolism). * **Acid-Labile Penicillins:** Penicillin G, Methicillin, and Carbenicillin (cannot be given orally). * **Drug of Choice:** Penicillin G remains the drug of choice for Gas gangrene (*C. perfringens*), Syphilis (*T. pallidum*), and Meningococcal meningitis. * **Jarisch-Herxheimer Reaction:** A classic systemic reaction (fever, chills) seen after starting penicillin for syphilis due to the release of endotoxins from dying spirochetes.

Question 648: Fluconazole is the drug of choice for which of the following fungal infections?

A. Candidiasis
B. Cryptococcus
C. Coccidioidomycosis
D. All of the above (Correct Answer)

Explanation: Fluconazole is a second-generation triazole antifungal that inhibits the enzyme **14-α-demethylase**, preventing the conversion of lanosterol to ergosterol. It is highly favored in clinical practice due to its excellent oral bioavailability, high water solubility, and superior penetration into the cerebrospinal fluid (CSF) [1], [2]. **Why "All of the above" is correct:** * **Candidiasis:** Fluconazole is the first-line agent for mucosal (oropharyngeal/esophageal) and vaginal candidiasis [1], [4]. It is also used for non-neutropenic systemic candidemia [4]. * **Cryptococcus:** While Amphotericin B + Flucytosine is the treatment of choice for the *induction* phase of Cryptococcal meningitis, **Fluconazole is the drug of choice for the maintenance/consolidation phase** and for long-term prophylaxis in HIV patients [3], [4]. * **Coccidioidomycosis:** Fluconazole is the preferred agent for both pulmonary and meningeal coccidioidomycosis due to its high CSF concentrations [4]. **Clinical Pearls for NEET-PG:** 1. **CSF Penetration:** Fluconazole has the highest CSF-to-serum ratio among azoles (>70-80%), making it the drug of choice for fungal meningitis maintenance [2]. 2. **Metabolism:** Unlike other azoles, Fluconazole is primarily excreted **unchanged in the urine**, making it effective for fungal UTIs but requiring dose adjustment in renal failure [1], [2]. 3. **Spectrum Limitation:** It has **no activity** against *Aspergillus* (Voriconazole is DOC) or Mucormycosis (Amphotericin B is DOC) [4]. 4. **Resistance:** *Candida krusei* is intrinsically resistant to Fluconazole, and *Candida glabrata* shows dose-dependent resistance.

Question 649: What is true regarding clavulanic acid?

A. Deactivates beta-lactamase (Correct Answer)
B. Decreases renal excretion of amoxicillin
C. Potentiates the action of penicillin
D. Decreases the side effects of amoxicillin

Explanation: ### Explanation **Correct Option: A (Deactivates beta-lactamase)** Clavulanic acid is a **suicide inhibitor** of the enzyme beta-lactamase. It contains a beta-lactam ring that binds irreversibly to the active site of the enzyme produced by resistant bacteria. By "sacrificing" itself to neutralize the enzyme, it prevents the degradation of the co-administered antibiotic (like Amoxicillin), thereby restoring its antibacterial spectrum. **Analysis of Incorrect Options:** * **Option B:** Clavulanic acid does not affect the renal excretion of amoxicillin. This is a characteristic of **Probenecid**, which inhibits the tubular secretion of penicillins to increase their plasma half-life. * **Option C:** While it allows amoxicillin to work against resistant strains, it does not "potentiate" the intrinsic pharmacological action of penicillin itself. It is a **pharmacokinetic protector** rather than a synergistic enhancer of the mechanism of action. * **Option D:** Clavulanic acid actually **increases** side effects, particularly gastrointestinal distress and diarrhea, compared to amoxicillin alone. **High-Yield NEET-PG Pearls:** 1. **Suicide Inhibitors:** Other examples include Sulbactam and Tazobactam. 2. **Spectrum:** Clavulanic acid is effective against Class A beta-lactamases (e.g., *S. aureus*, *H. influenzae*, *N. gonorrhoeae*) but is **ineffective** against AmpC or Metallo-beta-lactamases (NDM-1). 3. **Fixed-Dose Combination:** Amoxicillin + Clavulanic acid (Co-amoxiclav) is a classic example of overcoming drug resistance. 4. **Side Effect:** Augmentin (Amoxicillin/Clavulanate) is a common cause of drug-induced cholestatic jaundice.

Question 650: Which of the following medications shortens the recovery period of primary herpetic gingivostomatitis?

A. Acyclovir (Correct Answer)
B. Zidovudine
C. Triamcinolone acetonide in orabase
D. All of the above

Explanation: ### Explanation **Correct Option: A. Acyclovir** Primary herpetic gingivostomatitis is the most common clinical manifestation of primary **Herpes Simplex Virus (HSV-1)** infection. **Acyclovir**, a guanosine analogue, is the drug of choice. It acts by inhibiting viral DNA polymerase after being phosphorylated by viral thymidine kinase. When administered within 72 hours of symptom onset, it significantly reduces the duration of viral shedding, shortens the time to crusting of lesions, and decreases the recovery period (fever and pain duration). **Why other options are incorrect:** * **B. Zidovudine (AZT):** This is a nucleoside reverse transcriptase inhibitor (NRTI) used in the treatment of **HIV/AIDS**. It has no clinical efficacy against the Herpes Simplex Virus. * **C. Triamcinolone acetonide in orabase:** This is a topical corticosteroid used for non-infectious inflammatory conditions like aphthous ulcers. In viral infections like HSV, **corticosteroids are contraindicated** as they can suppress the local immune response, potentially leading to the spread of the virus and worsening of the infection (e.g., herpetic keratitis). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Resistance:** Resistance to Acyclovir most commonly occurs due to the absence or partial production of the enzyme **viral thymidine kinase**. * **Drug of Choice for HSV Encephalitis:** Intravenous Acyclovir. * **Valacyclovir vs. Acyclovir:** Valacyclovir is a prodrug of acyclovir with much higher oral bioavailability, allowing for less frequent dosing. * **Safety:** Acyclovir is generally safe in pregnancy (Category B). The most significant side effect of IV administration is **crystalline nephropathy**; ensure adequate hydration to prevent this.

Practice by Chapter

Beta-Lactam Antibiotics

Practice Questions

Aminoglycosides

Practice Questions

Macrolides and Ketolides

Practice Questions

Tetracyclines

Practice Questions

Quinolones

Practice Questions

Sulfonamides and Trimethoprim

Practice Questions

Antimycobacterial Drugs

Practice Questions

Antifungal Agents

Practice Questions

Antiviral Drugs

Practice Questions

Antiparasitic Agents

Practice Questions

Principles of Antimicrobial Selection

Practice Questions

Antimicrobial Resistance

Practice Questions

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