Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 721: Which of the following neuraminidase inhibitors has 100% renal elimination?

A. Oseltamivir
B. Zanamivir (Correct Answer)
C. Peramivir
D. Acyclovir

Explanation: **Explanation:** The question focuses on the pharmacokinetic profiles of neuraminidase inhibitors, which are essential for treating Influenza A and B. **1. Why Zanamivir is Correct:** **Zanamivir** is a potent neuraminidase inhibitor that is administered via inhalation (Rotadisk) or IV. It is not metabolized by the body and is excreted **100% unchanged in the urine** via glomerular filtration. Because it bypasses hepatic metabolism entirely, it has a pure renal elimination profile. **2. Analysis of Incorrect Options:** * **Oseltamivir (Option A):** This is a **prodrug** administered orally. It must undergo extensive hepatic metabolism by carboxylesterases to be converted into its active form, oseltamivir carboxylate. While the active metabolite is eventually excreted renally, the parent drug requires hepatic activation, making "100% renal elimination" incorrect. * **Peramivir (Option C):** While Peramivir is primarily eliminated renally (approx. 90%), it is an IV formulation used for acute uncomplicated influenza. Zanamivir remains the classic textbook answer for absolute renal elimination without prior metabolism. * **Acyclovir (Option D):** This is a DNA polymerase inhibitor used for Herpes Simplex (HSV) and Varicella-Zoster (VZV), not a neuraminidase inhibitor. While it is primarily excreted renally, it does not fit the drug class specified in the question. **High-Yield Clinical Pearls for NEET-PG:** * **Zanamivir Contraindication:** Avoid in patients with **Asthma or COPD** due to the risk of severe bronchospasm (since it is inhaled). * **Oseltamivir Side Effects:** Most common are GI upset (nausea/vomiting) and rare neuropsychiatric events in children. * **Mechanism of Action:** Neuraminidase inhibitors prevent the release of new virions from infected host cells by inhibiting the cleavage of sialic acid residues. * **Timing:** These drugs are most effective when started within **48 hours** of symptom onset.

Question 722: Which drug is not effective against Helicobacter pylori?

A. Colloidal bismuth
B. Metronidazole
C. Amoxicillin
D. Erythromycin (Correct Answer)

Explanation: ### Explanation The correct answer is **Erythromycin**. **1. Why Erythromycin is the correct answer:** While *Helicobacter pylori* is a Gram-negative bacterium, it is inherently resistant to **Erythromycin** in vivo. Although Erythromycin shows some activity in vitro, it is unstable in the acidic environment of the stomach. In contrast, **Clarithromycin** (a newer macrolide) is a cornerstone of *H. pylori* therapy because it is more acid-stable, achieves higher mucosal concentrations, and has superior efficacy against the organism. **2. Why the other options are incorrect:** * **Colloidal Bismuth (A):** Bismuth subsalicylate or colloidal bismuth subcitrate has direct bactericidal effects on *H. pylori*, prevents bacterial adhesion to the gastric mucosa, and inhibits bacterial enzymes (urease). It is a key component of "Bismuth Quadruple Therapy." * **Metronidazole (B):** This nitroimidazole is effective against anaerobic bacteria and microaerophilic organisms like *H. pylori*. It is frequently used in patients allergic to penicillin or in quadruple therapy regimens. * **Amoxicillin (C):** This is a primary bactericidal agent used in "Standard Triple Therapy." Resistance to amoxicillin remains remarkably low, making it a reliable first-line choice. **3. NEET-PG High-Yield Pearls:** * **Standard Triple Therapy (7–14 days):** PPI + Amoxicillin + Clarithromycin (Mnemonic: **PAC**). * **Bismuth Quadruple Therapy:** PPI + Bismuth + Metronidazole + Tetracycline (Used if Clarithromycin resistance is >15%). * **Sequential Therapy:** 5 days of PPI + Amoxicillin, followed by 5 days of PPI + Clarithromycin + Tinidazole. * **Diagnosis:** The non-invasive "Gold Standard" for confirming eradication is the **Urea Breath Test (UBT)**.

Question 723: Isoniazid (INH) causes neuropathy. Which of the following conditions is NOT associated with an increased risk of INH-induced neuropathy?

A. Uremia
B. Hyperthyroidism (Correct Answer)
C. Diabetes mellitus
D. Poor nutrition

Explanation: **Explanation:** Isoniazid (INH) is a cornerstone of antitubercular therapy (ATT), but its most common dose-related side effect is **peripheral neuropathy**. **Mechanism:** INH is structurally similar to **Pyridoxine (Vitamin B6)**. It promotes the excretion of pyridoxine and inhibits the enzyme *pyridoxine phosphokinase*, which converts B6 to its active form (pyridoxal phosphate). This active form is essential for the synthesis of GABA; its deficiency leads to nerve damage. **Why Hyperthyroidism is the Correct Answer:** * **Hyperthyroidism (Option B):** There is no established clinical or biochemical link between hyperthyroidism and increased susceptibility to INH-induced nerve damage. While it increases metabolic rate, it does not specifically exacerbate the B6-depletion mechanism. **Why the other options are associated with risk:** * **Uremia (Option A):** Renal failure leads to the accumulation of toxic metabolites and often coexists with metabolic disturbances that sensitize peripheral nerves to drug toxicity. * **Diabetes Mellitus (Option C):** Diabetics already have a baseline risk of "stocking-and-glove" neuropathy. INH-induced B6 deficiency acts synergistically with diabetic microvascular damage to worsen nerve function. * **Poor Nutrition (Option D):** Chronic malnutrition or alcoholism implies low baseline stores of Vitamin B6, making the patient highly vulnerable to further depletion by INH. **High-Yield Clinical Pearls for NEET-PG:** * **Slow Acetylators:** INH is metabolized by **Acetylation (NAT2 enzyme)**. Slow acetylators are at a much higher risk of neuropathy due to higher plasma concentrations of the drug. * **Prophylaxis:** To prevent neuropathy, INH is co-administered with **Pyridoxine (10–50 mg/day)**. * **Other Risk Groups:** Pregnancy, chronic alcoholism, and HIV infection are also high-risk categories for INH-induced neuropathy.

Question 724: Which of the following drugs used in multidrug-resistant tuberculosis (MDR TB) may lead to suicidal tendencies?

A. Para-aminosalicylic acid (PAS)
B. Pyrazinamide
C. Cycloserine (Correct Answer)
D. None of the above

Explanation: **Explanation:** **Cycloserine** is a second-line antitubercular drug used in the treatment of Multidrug-Resistant Tuberculosis (MDR-TB). It acts by inhibiting the enzymes D-alanine-D-alanine ligase and alanine racemase, thereby preventing bacterial cell wall synthesis. **Why Cycloserine is the correct answer:** Cycloserine is notorious for its significant **neuropsychiatric side effects**, often referred to as "Cycloserine Psychosis." It can cause a wide spectrum of CNS disturbances, including: * Severe depression and **suicidal ideation/tendencies**. * Psychosis, hallucinations, and acute anxiety. * Seizures and peripheral neuropathy. These effects occur because cycloserine is a structural analog of D-alanine and acts as a partial agonist at the NMDA (N-methyl-D-aspartate) receptors in the brain. **Why other options are incorrect:** * **Para-aminosalicylic acid (PAS):** Primarily causes severe gastrointestinal distress (nausea, vomiting, diarrhea) and hypothyroidism. It does not typically cause psychiatric symptoms. * **Pyrazinamide:** A first-line drug known for causing hepatotoxicity and hyperuricemia (which may lead to gouty arthritis), but it is not associated with suicidal tendencies. **High-Yield Clinical Pearls for NEET-PG:** * **Antidote:** To minimize the neurotoxic effects of Cycloserine, it is co-administered with **Pyridoxine (Vitamin B6)** (usually 50 mg for every 250 mg of cycloserine). * **Contraindications:** Cycloserine is strictly contraindicated in patients with a history of epilepsy, depression, or severe psychosis. * **Monitoring:** Patients on MDR-TB regimens containing Cycloserine must be closely monitored for "behavioral changes" at every follow-up visit.

Question 725: What is the recommended weekly dose of chloroquine for prophylaxis against vivax malaria?

A. 150 mg (base) weekly
B. 300 mg (base) weekly (Correct Answer)
C. 600 mg (base) weekly
D. 900 mg (base) weekly

Explanation: **Explanation:** **1. Why Option B is Correct:** Chloroquine remains the drug of choice for prophylaxis against malaria in regions where *Plasmodium vivax* and chloroquine-sensitive *P. falciparum* are prevalent. The standard prophylactic dose is **300 mg of chloroquine base (equivalent to 500 mg of chloroquine phosphate)** taken orally once a week. To achieve steady-state plasma concentrations, prophylaxis should ideally start 1–2 weeks before entering the endemic area and must continue for 4 weeks after leaving it. **2. Why the Other Options are Incorrect:** * **Option A (150 mg):** This is a sub-therapeutic dose for prophylaxis and would lead to breakthrough infections. 150 mg base is the amount typically found in a single standard tablet, but the weekly dose requires two such tablets. * **Option C (600 mg):** This is the **loading dose** used in the treatment of an acute attack of malaria (followed by 300 mg at 6, 24, and 48 hours), not for weekly prophylaxis. * **Option D (900 mg):** This dose exceeds the recommended prophylactic window and increases the risk of toxicity (retinopathy and gastrointestinal distress) without added benefit. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Chloroquine inhibits the enzyme **heme polymerase**, leading to the accumulation of toxic heme (ferriprotoporphyrin IX) which kills the parasite. * **Safety in Pregnancy:** Chloroquine is considered **safe during pregnancy**, making it the preferred prophylactic agent for pregnant women in sensitive zones. * **Adverse Effects:** Long-term use requires monitoring for **bull’s eye maculopathy** (retinal toxicity). * **Contraindication:** It should be avoided in patients with G6PD deficiency (risk of hemolysis) and those with psoriasis (can exacerbate skin lesions).

Question 726: Which of the following fluoroquinolones can be used safely in renal failure?

A. Ciprofloxacin
B. Ofloxacin
C. Lomefloxacin
D. Pefloxacin (Correct Answer)

Explanation: **Explanation:** The correct answer is **Pefloxacin**. **1. Why Pefloxacin is correct:** Most fluoroquinolones are primarily excreted unchanged by the kidneys via glomerular filtration and tubular secretion. Therefore, their dosage must be adjusted in patients with renal impairment. **Pefloxacin** (along with **Moxifloxacin**) is a notable exception because it undergoes extensive **hepatic metabolism**. It is metabolized in the liver to its active metabolite, norfloxacin, and other inactive metabolites. Since its clearance is not significantly dependent on renal function, it can be used safely in renal failure without major dose adjustments. **2. Why the other options are incorrect:** * **Ciprofloxacin:** Primarily excreted renally (approx. 50-70%). Accumulation occurs in renal failure, necessitating dose reduction. * **Ofloxacin:** Almost entirely excreted unchanged in the urine (>80%). It requires significant dose modification in patients with a low creatinine clearance. * **Lomefloxacin:** Primarily eliminated by the kidneys. Like most older fluoroquinolones, it is contraindicated or requires careful monitoring in renal insufficiency. **3. NEET-PG High-Yield Pearls:** * **Moxifloxacin** is another fluoroquinolone that does **not** require dose adjustment in renal failure (primarily biliary excretion). * Because Moxifloxacin and Pefloxacin have low urinary concentrations, they are generally **not** preferred for treating Urinary Tract Infections (UTIs). * **Levofloxacin** is the L-isomer of Ofloxacin and also requires dose adjustment in renal failure. * **Pefloxacin** is known for having the highest penetration into the Cerebrospinal Fluid (CSF) among fluoroquinolones.

Question 727: Which of the following protein synthesis inhibitors is a non-plant toxin?

A. Streptomycin (Correct Answer)
B. Abrin
C. Ricin
D. Emetine

Explanation: This question tests the ability to differentiate between therapeutic antimicrobial agents and naturally occurring biological toxins that share a common mechanism: the inhibition of protein synthesis. ### **Explanation of the Correct Answer** **Streptomycin (Option A)** is an **aminoglycoside antibiotic** derived from the soil bacterium *Streptomyces griseus*. It inhibits protein synthesis by binding to the **30S ribosomal subunit** of bacteria, causing misreading of mRNA and inhibition of the initiation complex. Unlike the other options, it is a clinically used pharmaceutical agent and is **not a plant toxin**. ### **Analysis of Incorrect Options** * **Abrin (Option B):** A highly potent toxalbumin found in the seeds of the **Rosary Pea (*Abrus precatorius*)**. It is a Type II Ribosome-Inactivating Protein (RIP) that inhibits the 60S ribosomal subunit by removing adenine from rRNA. * **Ricin (Option C):** A lethal toxin derived from the **Castor bean (*Ricinus communis*)**. Similar to abrin, it is a plant-derived RIP that halts protein synthesis at the 60S subunit, leading to cell death. * **Emetine (Option D):** An alkaloid derived from the **Ipecacuanha plant**. While historically used as an anti-amoebic, it acts by inhibiting protein synthesis in eukaryotic cells (40S subunit). It is considered a plant-derived alkaloid/toxin rather than a standard antibiotic. ### **High-Yield Clinical Pearls for NEET-PG** * **Mechanism of Streptomycin:** It is bactericidal (unlike most protein synthesis inhibitors which are bacteriostatic) and is a first-line drug for **Tuberculosis**. * **Adverse Effects:** Ototoxicity (vestibular > auditory) and Nephrotoxicity. * **Ribosomal Targets:** * **30S Inhibitors:** Aminoglycosides, Tetracyclines. * **50S Inhibitors:** Chloramphenicol, Macrolides, Clindamycin, Linezolid. * **60S (Eukaryotic) Inhibitors:** Ricin, Abrin, Shiga toxin.

Question 728: All of the following are beta-lactamase inhibitors except?

A. Clavulanic acid
B. Sulbactam
C. Tazobactam
D. Aztreonam (Correct Answer)

Explanation: ### Explanation The correct answer is **D. Aztreonam**. **1. Why Aztreonam is the correct answer:** Aztreonam is a **Monobactam**, which is a class of beta-lactam antibiotics. Unlike beta-lactamase inhibitors, which have little to no intrinsic antibacterial activity and are used to protect other antibiotics, Aztreonam is a potent bactericidal agent. It specifically targets **Gram-negative aerobic bacteria** (including *Pseudomonas*) by binding to penicillin-binding protein 3 (PBP-3). It is unique because it is resistant to many beta-lactamases but is **not** an inhibitor itself. **2. Why the other options are incorrect:** * **Clavulanic acid, Sulbactam, and Tazobactam** are classic **"Suicide Inhibitors."** They contain a beta-lactam ring but possess negligible antimicrobial activity. Instead, they bind irreversibly to the beta-lactamase enzymes produced by bacteria, preventing the enzymes from destroying co-administered antibiotics like Amoxicillin, Ampicillin, or Piperacillin. **3. High-Yield Clinical Pearls for NEET-PG:** * **Aztreonam's "Safety Profile":** It is the only beta-lactam that shows **no cross-reactivity** with penicillins (except for Ceftazidime due to a similar side chain). It is the drug of choice for Gram-negative infections in patients with a severe penicillin allergy. * **Newer Beta-lactamase Inhibitors:** Beyond the options listed, remember **Avibactam, Relebactam, and Vaborbactam**. Unlike the older "suicide inhibitors," Avibactam does not have a beta-lactam ring (it is a diazabicyclooctane). * **Spectrum:** Beta-lactamase inhibitors are typically effective against Class A beta-lactamases (like those from *S. aureus*, *H. influenzae*) but are often ineffective against AmpC or Carbapenemases (except for the newer agents like Avibactam).

Question 729: Which of the following is not nephrotoxic?

A. Tobramycin
B. Kanamycin
C. Ampicillin (Correct Answer)
D. Amphotericin B

Explanation: ### Explanation The correct answer is **Ampicillin**. **1. Why Ampicillin is the correct choice:** Ampicillin is a penicillin-class antibiotic. Penicillins are generally considered safe for the kidneys because they are primarily excreted via active tubular secretion without causing direct damage to the renal parenchyma. While they can rarely cause **Acute Interstitial Nephritis (AIN)**—an immune-mediated hypersensitivity reaction—they are not inherently **nephrotoxic** (toxic to renal cells). **2. Why the other options are incorrect:** * **Tobramycin & Kanamycin (Aminoglycosides):** These are notorious for causing **Acute Tubular Necrosis (ATN)**. They accumulate in the proximal convoluted tubule (PCT) cells, leading to dose-dependent, reversible renal failure. Neomycin is the most nephrotoxic, while Streptomycin is the least. * **Amphotericin B:** This antifungal is highly nephrotoxic. It causes renal vasoconstriction and direct damage to the distal tubular membranes, often leading to "Type 1 Renal Tubular Acidosis" and significant electrolyte wasting (hypokalemia and hypomagnesemia). **3. NEET-PG High-Yield Clinical Pearls:** * **Aminoglycoside Nephrotoxicity:** It is usually non-oliguric. Monitoring serum creatinine is essential. * **Amphotericin B:** To reduce its nephrotoxicity, clinicians use **Liposomal formulations** or "salt loading" (pre-infusion with normal saline). * **Drug of Choice for AIN:** If a patient develops hematuria and eosinophiluria after taking Ampicillin or Methicillin, the diagnosis is AIN; the immediate step is to stop the drug. * **Other common nephrotoxic drugs:** Cisplatin, Cyclosporine, NSAIDs, and Radiocontrast dyes.

Question 730: All of the following drugs act on nucleic acids except?

A. Fluoroquinolones
B. Linezolid (Correct Answer)
C. Rifampicin
D. Nalidixic acid

Explanation: **Explanation:** The core concept tested here is the classification of antimicrobial agents based on their site of action. **Why Linezolid is the correct answer:** Linezolid belongs to the **Oxazolidinone** class. Its mechanism of action is the **inhibition of protein synthesis**. Specifically, it binds to the 23S ribosomal RNA of the **50S subunit**, preventing the formation of the functional 70S initiation complex. It does not directly interact with DNA or RNA synthesis. **Why the other options are incorrect:** * **Fluoroquinolones (e.g., Ciprofloxacin):** These act directly on nucleic acids by inhibiting **DNA Gyrase** (Topoisomerase II) and **Topoisomerase IV**, thereby preventing DNA replication and transcription. * **Nalidixic acid:** This is a first-generation quinolone. Like fluoroquinolones, it acts by inhibiting DNA Gyrase, targeting bacterial DNA synthesis. * **Rifampicin:** This drug inhibits **DNA-dependent RNA polymerase**, thereby blocking the synthesis of messenger RNA (mRNA). Since it targets the transcription phase of nucleic acid metabolism, it is classified as acting on nucleic acids. **High-Yield Clinical Pearls for NEET-PG:** * **Linezolid:** It is a "reserve drug" primarily used for **MRSA** (Methicillin-resistant *Staphylococcus aureus*) and **VRE** (Vancomycin-resistant *Enterococci*). * **Side Effects of Linezolid:** Long-term use can cause **thrombocytopenia** (bone marrow suppression) and **optic/peripheral neuropathy**. * **Drug Interaction:** Linezolid is a weak MAO inhibitor; it can precipitate **Serotonin Syndrome** if co-administered with SSRIs. * **Mnemonic for Protein Synthesis Inhibitors:** "**Buy AT 30, CELL at 50**" (Aminoglycosides, Tetracyclines act on 30S; Chloramphenicol, Erythromycin/Macrolides, Linezolid, Lincosamides act on 50S).

Practice by Chapter

Beta-Lactam Antibiotics

Practice Questions

Aminoglycosides

Practice Questions

Macrolides and Ketolides

Practice Questions

Tetracyclines

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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

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