Antimicrobial Agents Practice Questions

Q: What is the recommended single dose of fosfomycin for a 40-year-old patient weighing 60 kg?

3 g. Fosfomycin is a unique bactericidal antibiotic that inhibits the first step of bacterial cell wall synthesis by inactivating the enzyme Enolpyruvate transferase (MurA). This prevents the formation of UDP-N-acetylmuramic acid, a precursor of peptidoglycan. 1. Why 3 g is correct: For the treatment of uncomplicated urinary tract infections (UTIs), specifically acute cystitis in women caused by susceptible strains of E. coli or Enterococcus faecalis, the standard FDA-approved and clinical guideline dosage is a single 3-gram oral dose. It is administered as granules dissolved in water. The drug achieves very high concentrations in the urine (up to 2000–4000 µg/mL) which remain above the MIC for most uropathogens for 24–48 hours, justifying the single-dose regimen. 2. Why other options are incorrect: 6 g (Option A): This is double the standard dose for uncomplicated UTI and is not a standard single-dose regimen. 6 mg/kg and 3 mg/kg (Options B & D): Fosfomycin dosing for UTIs is fixed, not weight-based. Weight-based dosing (mg/kg) is typically reserved for intravenous fosfomycin used in systemic or multidrug-resistant infections, but even then, the doses are significantly higher (e.g., 100–400 mg/kg/day). High-Yield Clinical Pearls for NEET-PG: Mechanism of Action: Inhibits MurA enzyme (Cell wall synthesis inhibitor). Spectrum: Broad-spectrum (Gram-positive and Gram-negative); notably effective against VRE (Vancomycin-Resistant Enterococci) and ESBL-producing organisms. Pregnancy: It is considered safe (Category B) and is often used for asymptomatic bacteriuria in pregnancy. Synergy: Often shows synergy when combined with Beta-lactams or Aminoglycosides.

Q: Isoniazid may show cross-resistance with which of the following anti-tubercular drugs?

Ethionamide. **Explanation:** The correct answer is **Ethionamide**. **Mechanism of Cross-Resistance:** The cross-resistance between Isoniazid (INH) and Ethionamide is rooted in their shared mechanism of action. Both drugs are **prodrugs** that inhibit **mycolic acid synthesis**, a vital component of the mycobacterial cell wall. * INH is activated by the enzyme **KatG** (catalase-peroxidase) and targets the **InhA** enzyme (enoyl-ACP reductase). * Ethionamide is activated by a different enzyme (**EthA**) but targets the **same InhA enzyme**. * **The Link:** Mutations in the **inhA gene** (promoter region) lead to overexpression of the InhA enzyme. Since both drugs target this specific enzyme, a mutation here results in **cross-resistance** between INH and Ethionamide. (Note: Mutations in the *katG* gene cause high-level resistance to INH only, without affecting Ethionamide). **Why Incorrect Options are Wrong:** * **Rifampicin:** Acts by inhibiting DNA-dependent RNA polymerase (*rpoB* gene mutation). It has a completely different target and activation pathway. * **Cycloserine:** A second-line drug that inhibits peptidoglycan synthesis by acting as an antimetabolite of D-alanine. * **Streptomycin:** An aminoglycoside that inhibits protein synthesis by binding to the 30S ribosomal subunit (*rpsL* or *rrs* gene mutations). **High-Yield Clinical Pearls for NEET-PG:** * **INH Resistance:** Most commonly due to *katG* gene mutation (high-level resistance) or *inhA* gene mutation (low-level resistance + cross-resistance with Ethionamide). * **Ethionamide Side Effects:** Notable for causing intense GI irritation and a metallic taste. * **Fast vs. Slow Acetylators:** INH metabolism depends on N-acetyltransferase 2 (NAT2). Slow acetylators are prone to peripheral neuropathy (prevented by **Pyridoxine/Vit B6**), while fast acetylators may show poor response.

Q: Which antimicrobial agent is effective against ESBL-producing microorganisms?

Beta-lactam plus beta-lactamase inhibitor. Explanation: Extended-Spectrum Beta-Lactamases (ESBLs) are enzymes produced by certain bacteria (most commonly *E. coli* and *Klebsiella*) that mediate resistance by hydrolyzing the beta-lactam ring [2]. 1. Why Option A is Correct: ESBLs are typically inhibited by Beta-lactamase inhibitors (BLIs) such as Clavulanic acid, Sulbactam, and Tazobactam. Therefore, combinations like Piperacillin-Tazobactam or Cefoperazone-Sulbactam remain effective. However, note that Carbapenems (e.g., Meropenem) are clinically considered the "gold standard" treatment for serious ESBL infections [1]. 2. Why Options B, C, and D are Incorrect: * Ampicillin (B): ESBLs readily hydrolyze penicillins. * Ceftriaxone (C): By definition, ESBLs confer resistance to all 1st, 2nd, and 3rd generation cephalosporins (like Ceftriaxone and Ceftazidime) as well as 4th generation cephalosporins (Cefepime) [1][3]. * Aztreonam (D): ESBLs also hydrolyze Monobactams [1]. Aztreonam is only stable against Metallo-beta-lactamases (NDM-1), not ESBLs. High-Yield Clinical Pearls for NEET-PG: * The "Carbapenem Gap": While BLI combinations work *in vitro*, Carbapenems are the drugs of choice for ESBL-related urosepsis or bacteremia [1]. * Marker for ESBL: Resistance to Ceftazidime is often used as a laboratory screening marker for ESBL production. * Co-resistance: ESBL-producing organisms often carry genes for resistance to other classes, such as Fluoroquinolones and Aminoglycosides. * Exceptions: ESBLs do not hydrolyze Cephamycins (e.g., Cefoxitin, Cefotetan), though these are rarely used clinically for this purpose.

Q: What drug is associated with the following adverse drug reaction (ADR)?

Demeclocycline. ***Demeclocycline*** - A **tetracycline antibiotic** that causes **photosensitivity reactions** due to its ability to absorb UV light and generate reactive oxygen species in the skin. - The **phototoxic dermatitis** appears as sunburn-like lesions on **sun-exposed areas** such as face, neck, and hands. *Chloramphenicol* - Primary adverse effects include **bone marrow suppression** leading to aplastic anemia and **gray baby syndrome** in neonates. - **Not associated** with photosensitivity reactions; its toxicity is mainly hematological and hepatic. *Erythromycin* - Commonly causes **gastrointestinal upset** (nausea, vomiting, diarrhea) and **hepatotoxicity** with prolonged use. - **Photosensitivity** is not a recognized adverse effect of this macrolide antibiotic. *Daptomycin* - Major adverse effects include **myopathy** with elevated creatine kinase levels and **eosinophilic pneumonia**. - **No association** with photosensitivity; monitoring focuses on muscle toxicity and pulmonary symptoms.

Q: Which drug predisposes to pyloric stenosis in infants?

Erythromycin. **Explanation:** The correct answer is **Erythromycin**. **Why Erythromycin is correct:** Erythromycin, a macrolide antibiotic, is a known risk factor for **Infantile Hypertrophic Pyloric Stenosis (IHPS)**. The underlying mechanism involves its action as a **motilin receptor agonist**. Erythromycin mimics the action of the hormone motilin, which stimulates gastrointestinal motility. In neonates (especially those under 2 weeks of age), excessive stimulation of these receptors leads to strong, uncoordinated contractions of the pyloric antrum, eventually resulting in hypertrophy of the pyloric sphincter and gastric outlet obstruction. This risk is highest with oral erythromycin but has also been associated with maternal use during late pregnancy or breastfeeding. **Why the other options are incorrect:** * **Streptomycin:** An aminoglycoside primarily associated with **ototoxicity** (8th cranial nerve damage) and **nephrotoxicity**. In utero exposure can lead to congenital deafness. * **Tetracycline:** Known for causing **discoloration of teeth** and enamel hypoplasia in children due to its ability to chelate calcium. It also inhibits bone growth. * **Nitrofurantoin:** Primarily associated with **hemolytic anemia** in neonates if used near term, especially in those with G6PD deficiency, but it does not affect the pylorus. **Clinical Pearls for NEET-PG:** * **Azithromycin** also carries a risk of IHPS, though it is statistically lower than Erythromycin. * **Classic Presentation of IHPS:** Non-bilious, projectile vomiting in an infant (3–6 weeks old) with a palpable "olive-shaped" mass in the epigastrium. * **Metabolic Abnormality in IHPS:** Hypochloremic, hypokalemic metabolic alkalosis (due to loss of HCl from vomiting). * **Drug of choice for Gastroparesis:** Erythromycin is sometimes used off-label in adults to stimulate gastric emptying due to its prokinetic properties.

Question 1

What is the recommended single dose of fosfomycin for a 40-year-old patient weighing 60 kg?

Accepted Answer

3 g

Answer

6 g

Answer

6 mg/kg

Answer

3 mg/kg

Question 2

All of the following are true about Penicillins except?

Accepted Answer

Benzathine penicillin is used in the treatment of neurosyphilis.

Answer

Penicillin V is acid stable and can be given orally.

Answer

Probenecid increases the duration of action of penicillins.

Answer

Penicillin V acts mainly on Gram-positive microorganisms.

Question 3

Isoniazid may show cross-resistance with which of the following anti-tubercular drugs?

Accepted Answer

Ethionamide

Answer

Rifampicin

Answer

Cycloserine

Answer

Streptomycin

Question 4

Which of the following best describes the mechanism of action of nucleoside/nucleotide analogs, such as didanosine, used in highly active antiretroviral therapy (HAART) for HIV patients?

Accepted Answer

They prematurely terminate the DNA synthesized by reverse transcriptase.

Answer

They prevent the hydrolysis of the viral polyprotein.

Answer

They directly bind to and inhibit reverse transcriptase.

Answer

They inhibit the synthesis of viral proteins.

Question 5

Which antimicrobial agent is effective against ESBL-producing microorganisms?

Accepted Answer

Beta-lactam plus beta-lactamase inhibitor

Answer

Ampicillin

Answer

Ceftriaxone

Answer

Aztreonam

Question 6

What drug is associated with the following adverse drug reaction (ADR)?

Accepted Answer

Demeclocycline

Answer

Chloramphenicol

Answer

Erythromycin

Answer

Daptomycin

Question 7

All of the following drugs are primarily bacteriostatic except?

Accepted Answer

Rifampicin

Answer

Sulphonamide

Answer

Chloramphenicol

Answer

Ethambutol

Question 8

Which class of antiretroviral drug is used for starting treatment for HIV infection?

Accepted Answer

Nucleoside Reverse Transcriptase Inhibitors (NRTIs)

Answer

Protease inhibitors

Answer

Fusion inhibitors

Answer

Non-Nucleoside Reverse Transcriptase Inhibitors (NNRTIs)

Question 9

Which drug predisposes to pyloric stenosis in infants?

Accepted Answer

Erythromycin

Answer

Streptomycin

Answer

Tetracycline

Answer

Nitrofurantoin

Question 10

Which of the following statements regarding amantadine and rimantadine is false?

Accepted Answer

Combination of amantadine and rimantadine are effective on Influenza B.

Answer

Rimantadine is 4-10 times more active than amantadine.

Answer

Both act by inhibiting an early step in viral replication.

Answer

They are effective against influenza A H1N1 if treatment is initiated within 2 days of the onset of symptoms.

Antimicrobial Agents — MCQs

Antimicrobial Agents — MCQs

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