Antimicrobial Agents Practice Questions

Q: Which antitubercular agent blocks RNA transcription?

Rifampicin. **Explanation:** **Correct Answer: A. Rifampicin** Rifampicin is a bactericidal antitubercular drug that acts by inhibiting the **DNA-dependent RNA polymerase** enzyme. By binding to the beta-subunit of this enzyme, it prevents the initiation of RNA synthesis, thereby blocking **RNA transcription**. This mechanism is highly specific to bacterial enzymes, which accounts for its selective toxicity. **Analysis of Incorrect Options:** * **B. Streptomycin:** This is an aminoglycoside that acts by binding to the **30S ribosomal subunit**. It inhibits **protein synthesis** by causing misreading of mRNA and interfering with the initiation complex. * **C. Chloramphenicol:** This is a broad-spectrum antibiotic (not a primary antitubercular agent) that inhibits **protein synthesis** by binding to the **50S ribosomal subunit** and blocking the enzyme peptidyl transferase. * **D. Isoniazid (INH):** This is a prodrug activated by the enzyme *KatG*. It inhibits the synthesis of **mycolic acids**, which are essential components of the mycobacterial cell wall. **High-Yield Clinical Pearls for NEET-PG:** * **Resistance:** Resistance to Rifampicin occurs due to mutations in the **rpoB gene** (encoding the beta-subunit of RNA polymerase). * **Side Effects:** A classic "exam favorite" side effect is the **orange-red discoloration** of body fluids (urine, sweat, tears). It is also a potent **Cytochrome P450 inducer**, leading to numerous drug interactions (e.g., reducing the efficacy of oral contraceptives and warfarin). * **Mnemonic:** Remember **"R"** for **R**ifampicin, **R**NA polymerase inhibition, and **R**ed-orange secretions.

Q: Which of the following tetracyclines can be used in renal failure without dose adjustment?

Doxycycline. **Explanation:** The correct answer is **Doxycycline**. **Why Doxycycline is correct:** Most tetracyclines are primarily excreted by the kidneys via glomerular filtration. Therefore, in patients with renal impairment, these drugs accumulate, leading to increased toxicity (including an anti-anabolic effect that worsens azotemia). **Doxycycline** is the notable exception; it is primarily excreted as an inactive chelate via the **feces (biliary excretion)**. Because its clearance is independent of renal function, it is the safest tetracycline for patients with renal failure and requires no dose adjustment. **Why the other options are wrong:** * **Oxytetracycline:** This is a short-acting tetracycline that is highly dependent on renal excretion. It is contraindicated in renal failure. * **Minocycline:** While it undergoes significant hepatic metabolism, it still relies partly on renal clearance. While safer than oxytetracycline, Doxycycline remains the preferred "gold standard" for renal safety. * **Demeclocycline:** This intermediate-acting agent is excreted renally. Notably, it is used clinically to treat SIADH because it induces nephrogenic diabetes insipidus—a side effect that would be dangerous in a patient with pre-existing renal compromise. **NEET-PG High-Yield Pearls:** * **Drug of Choice:** Doxycycline is the drug of choice for Chlamydia, Rickettsial infections (Rocky Mountain Spotted Fever), and Lyme disease. * **Storage:** Expired tetracyclines can cause **Fanconi Syndrome** (proximal renal tubular acidosis). * **Contraindications:** Avoid in pregnancy and children <8 years due to bone growth retardation and permanent tooth discoloration. * **Phototoxicity:** Demeclocycline has the highest incidence of phototoxic reactions among tetracyclines.

Q: What is the topical antibiotic of choice for MRSA?

Mupirocin. **Explanation:** **Mupirocin** is the topical antibiotic of choice for Methicillin-resistant *Staphylococcus aureus* (MRSA). Its unique mechanism of action involves the inhibition of **isoleucyl-tRNA synthetase**, which halts bacterial protein synthesis. Because this mechanism differs from other antibiotic classes, there is minimal cross-resistance. It is primarily used for the eradication of nasal carriage of MRSA in both patients and healthcare workers and is also the first-line treatment for impetigo caused by *S. aureus* or *S. pyogenes*. **Analysis of Incorrect Options:** * **Mafenide & Silver sulfadiazine:** These are topical sulfonamides primarily used in **burn patients**. While they have broad-spectrum activity, their main clinical utility is preventing *Pseudomonas* infections in burn wounds, not targeted MRSA eradication. Mafenide is notable for causing metabolic acidosis (via carbonic anhydrase inhibition). * **Butenafine:** This is a topical **antifungal** agent (benzylamine derivative) used for dermatophytosis (tinea infections). It has no antibacterial activity. **High-Yield Clinical Pearls for NEET-PG:** * **Mupirocin Resistance:** High-level resistance is mediated by the *mupA* gene. * **Nasal Decolonization:** The standard regimen is twice-daily application in the anterior nares for 5 days. * **Alternative for MRSA Decolonization:** If mupirocin resistance is present, topical **Retapamulin** (a pleuromutilin) or **Povidone-iodine** may be used. * **Systemic MRSA:** For systemic infections, the drugs of choice are Vancomycin, Linezolid, or Daptomycin.

Q: Which anti-tubercular drug is generally not given in patients with liver disease?

Pyrazinamide. **Explanation:** The management of tuberculosis in patients with pre-existing liver disease requires careful selection of drugs, as three out of the four first-line agents are hepatotoxic. **Why Pyrazinamide is the correct answer:** Pyrazinamide (PZA) is considered the **most hepatotoxic** of all first-line anti-tubercular drugs (ATDs). It causes dose-dependent hepatotoxicity and can lead to prolonged liver injury. According to standard guidelines (WHO/RNTCP), if a patient has unstable or advanced chronic liver disease, Pyrazinamide is the first drug to be excluded from the regimen to prevent fulminant hepatic failure. **Analysis of Incorrect Options:** * **Isoniazid (INH):** While INH is hepatotoxic (causing idiosyncratic hepatitis), it is often retained in modified regimens for liver disease unless the liver injury is severe, as its therapeutic benefit is high. * **Rifampicin:** It is also hepatotoxic and can cause transient cholestatic jaundice. However, it is less hepatotoxic than Pyrazinamide and is usually the backbone of "liver-safe" regimens (e.g., 2HRE + 7HR). * **Ethambutol:** This is the correct choice for patients with liver disease because it is **not hepatotoxic**. It is primarily excreted by the kidneys. **NEET-PG High-Yield Pearls:** 1. **Hepatotoxicity Order:** Pyrazinamide > Isoniazid > Rifampicin. 2. **Safe Drugs in Liver Disease:** Ethambutol and Streptomycin (neither is metabolized by the liver). 3. **Regimen for Stable Chronic Liver Disease:** If the liver is compensated, a regimen like **2HRE + 7HR** (avoiding Pyrazinamide) is commonly used. 4. **Monitoring:** If ALT/AST levels rise >3 times the upper limit of normal with symptoms, or >5 times without symptoms, all hepatotoxic drugs must be stopped immediately.

Q: Cidofovir can be used for which of the following conditions?

All of the above. **Explanation:** **Cidofovir** is a potent acyclic nucleoside phosphonate analog that acts as a broad-spectrum antiviral agent. Unlike acyclovir, it is already phosphorylated (a nucleotide analog), meaning it does not require activation by viral kinases (like thymidine kinase). This allows it to remain effective against resistant viral strains. 1. **Why "All of the above" is correct:** * **Respiratory Papillomatosis:** Cidofovir is highly effective against Human Papillomavirus (HPV). It is used off-label via intralesional injection to treat recurrent respiratory papillomatosis, significantly reducing the frequency of surgical debridements. * **Herpes Simplex (HSV) & Herpes Zoster (VZV):** While not first-line, Cidofovir is indicated for mucocutaneous HSV and VZV infections, particularly in immunocompromised patients where the virus has developed resistance to Acyclovir or Foscarnet. 2. **Analysis of Options:** * **Option A:** Correct. It inhibits HPV DNA polymerase and induces apoptosis in HPV-infected cells. * **Option B & C:** Correct. It inhibits the DNA polymerase of almost all DNA viruses, including the entire Herpesviridae family (HSV, VZV, CMV, EBV, and HHV-8). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Competitive inhibition of viral DNA polymerase; it incorporates into the growing DNA chain, causing chain termination. * **Primary Indication:** CMV retinitis in HIV/AIDS patients. * **Dose-Limiting Toxicity:** **Nephrotoxicity** (proximal tubular damage). * **Prevention of Toxicity:** Must be administered with **Oral Probenecid** and aggressive **IV Saline hydration** to reduce uptake into proximal tubule cells and minimize kidney damage. * **Spectrum:** Broad DNA virus coverage including Poxvirus (Molluscum contagiosum), Adenovirus, and Polyomavirus (BK virus).

Question 1

Which antitubercular agent blocks RNA transcription?

Accepted Answer

Rifampicin

Answer

Streptomycin

Answer

Chloramphenicol

Answer

Isoniazid

Question 2

Which of the following tetracyclines can be used in renal failure without dose adjustment?

Accepted Answer

Doxycycline

Answer

Oxytetracycline

Answer

Minocycline

Answer

Demeclocycline

Question 3

All of the following statements about penicillin-binding proteins are true EXCEPT:

Accepted Answer

Methicillin-resistant Staphylococcus aureus (MRSA) has acquired a PBP with very high affinity for beta-lactam antibiotics.

Answer

Present on the cell surface

Answer

Mutation in PBPs gives rise to resistance

Answer

They are target sites of vancomycin

Question 4

What is the topical antibiotic of choice for MRSA?

Accepted Answer

Mupirocin

Answer

Mafenide

Answer

Silver sulfadiazine

Answer

Butenafine

Question 5

Which anti-tubercular drug is generally not given in patients with liver disease?

Accepted Answer

Pyrazinamide

Answer

Isoniazid

Answer

Ethambutol

Answer

Rifampicin

Question 6

Tetracyclines are the first drug of choice for which of the following conditions EXCEPT?

Accepted Answer

Community-acquired pneumonia

Answer

Granuloma inguinale

Answer

Cholera

Answer

Atypical pneumonia

Question 7

Cidofovir can be used for which of the following conditions?

Accepted Answer

All of the above

Answer

Respiratory papillomatosis

Answer

Herpes simplex

Answer

Herpes zoster

Question 8

What is the recommended dose of Artemether for severe malaria?

Accepted Answer

2.4 mg/kg

Answer

1.2 mg/Kg

Answer

1.7 mg/kg

Answer

3.4 mg/kg

Question 9

What is the drug of choice for cerebral malaria caused by Plasmodium falciparum?

Accepted Answer

Quinine

Answer

Chloroquine

Answer

Mefloquine

Answer

Sulfadoxine + Pyrimethamine

Question 10

Which of the following is NOT a mechanism of resistance to erythromycin?

Accepted Answer

Erythromycin esterase production

Answer

Alteration in ribosomal binding site

Answer

Plasmid mediated

Answer

Efflux proteins

Antimicrobial Agents — MCQs

Antimicrobial Agents — MCQs

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