Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 901: What is the best treatment for roundworm infection?

A. Metronidazole
B. Mebendazole
C. Albendazole (Correct Answer)
D. Pyrantel pamoate

Explanation: **Explanation:** **Albendazole** is considered the drug of choice for roundworm (*Ascaris lumbricoides*) infections. Its mechanism of action involves inhibiting **microtubule synthesis** by binding to the colchicine-sensitive site of **β-tubulin**. This prevents the polymerization of microtubules, leading to impaired glucose uptake and eventual depletion of glycogen stores in the parasite, causing its death. **Why Albendazole is preferred:** While both Albendazole and Mebendazole are effective, Albendazole is preferred for mass drug administration and clinical practice because it is administered as a **single 400 mg dose**, ensuring better patient compliance compared to the multi-day regimens often required for other agents. **Analysis of Incorrect Options:** * **Metronidazole (A):** This is an antiprotozoal and anaerobic antibacterial agent. It is the drug of choice for Amoebiasis, Giardiasis, and Trichomoniasis, but it has no activity against helminths like roundworms. * **Mebendazole (B):** While effective against roundworms, it typically requires a twice-daily dose for three days. It is less preferred than the single-dose convenience of Albendazole. * **Pyrantel Pamoate (D):** This is a luminal agent that acts as a depolarizing neuromuscular blocker, causing spastic paralysis of the worm. While effective, it is generally considered a second-line alternative to benzimidazoles. **NEET-PG High-Yield Pearls:** * **Mechanism:** Benzimidazoles (Albendazole/Mebendazole) inhibit microtubule assembly. * **Drug of Choice (DOC):** Albendazole is the DOC for *Ascaris*, Hookworm, Enterobius (Pinworm), and Hydatid disease (*Echinococcus granulosus*). * **Pregnancy:** Albendazole is generally avoided in the first trimester due to potential embryotoxicity. * **Neurocysticercosis:** Albendazole is the preferred agent over Praziquantel due to better CNS penetration.

Question 902: What is the intraluminal amoebicide of choice?

A. Metronidazole
B. Diloxanide furoate (Correct Answer)
C. Chloroquine
D. Tetracycline

Explanation: **Explanation:** Amoebiasis treatment is categorized based on the site of action: **Luminal** (acting in the bowel lumen) and **Systemic** (acting in the intestinal wall and liver). **Why Diloxanide Furoate is correct:** Diloxanide furoate is a highly effective **luminal amoebicide**. It is poorly absorbed from the gastrointestinal tract, allowing high concentrations to remain in the gut lumen to kill *Entamoeba histolytica* cysts. It is the drug of choice for treating asymptomatic cyst passers and is used after a course of systemic amoebicides to eradicate any surviving organisms in the colon to prevent relapse. **Analysis of Incorrect Options:** * **Metronidazole:** This is a **systemic amoebicide**. While it is the drug of choice for invasive amoebiasis (amoebic dysentery and liver abscess), it is extensively absorbed from the small intestine, meaning it does not reach sufficient concentrations in the colon to reliably eradicate cysts. * **Chloroquine:** This is a **selective systemic amoebicide** that reaches high concentrations in the liver. It is used specifically for amoebic liver abscesses, usually when metronidazole is contraindicated or ineffective. It has no luminal activity. * **Tetracycline:** This is an **indirect amoebicide**. It acts by inhibiting the symbiotic bacterial flora in the gut that the amoebae depend on for nutrition. It is not a primary choice for intraluminal eradication. **NEET-PG High-Yield Pearls:** * **Luminal Amoebicides:** Diloxanide furoate, Paromomycin (an aminoglycoside), and Iodoquinol. * **Treatment Strategy:** For asymptomatic cyst passers, use a luminal agent alone. For invasive disease, always use a systemic agent (Metronidazole/Tinidazole) **followed by** a luminal agent. * **Drug of Choice for Pregnancy:** Paromomycin is preferred for luminal infections in pregnancy as it is not absorbed.

Question 903: Which of the following statement is not true regarding dapsone?

A. It acts by inhibiting folic acid synthesis
B. Poor oral absorption (Correct Answer)
C. It can cause DDS syndrome
D. Can cause hemolysis

Explanation: **Explanation:** Dapsone (Diaminodiphenyl sulfone) is the cornerstone of multidrug therapy (MDT) for leprosy. The statement "Poor oral absorption" is **incorrect** because Dapsone is actually **well-absorbed** from the gastrointestinal tract, with nearly 100% bioavailability. It is widely distributed in the body and undergoes enterohepatic circulation. **Analysis of other options:** * **Option A (Mechanism of Action):** Dapsone is a structural analog of PABA. It acts by competitively inhibiting the enzyme **dihydropteroate synthase**, thereby blocking folic acid synthesis in *M. leprae*. This is the same mechanism as sulfonamides. * **Option C (DDS Syndrome):** Also known as "Sulfone Syndrome," this is a severe hypersensitivity reaction occurring 4–6 weeks after starting therapy. It is characterized by fever, malaise, exfoliative dermatitis, jaundice, and lymphadenopathy. * **Option D (Hemolysis):** Dapsone causes oxidative stress. It can lead to dose-related hemolysis, especially in patients with **G6PD deficiency**. It is also frequently associated with methemoglobinemia. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** Dapsone is used for Leprosy, Dermatitis Herpetiformis, and *Pneumocystis jirovecii* prophylaxis. * **Metabolism:** It is metabolized via **acetylation** (similar to Isoniazid and Hydralazine). * **Side Effects:** Remember the triad of **Hemolysis, Methemoglobinemia, and Sulfone Syndrome.** * **Lepra Reactions:** Dapsone is continued during Type 1 and Type 2 lepra reactions.

Question 904: Methenamine salts are used as urinary antiseptics. The reason they lack systemic antibacterial action is that they are :

A. Not absorbed into systemic circulation after oral use
B. Rapidly metabolized by liver drug metabolizing enzymes
C. Converted to formaldehyde at low urinary pH (Correct Answer)
D. Substrates for active tubular secretion

Explanation: ### Explanation **Concept:** Methenamine (hexamethylene tetramine) is a **prodrug** that lacks intrinsic antibacterial activity. Its mechanism of action is entirely dependent on its decomposition into **formaldehyde** and ammonia. This chemical conversion occurs only in an **acidic environment (pH < 5.5)**. Since physiological blood pH is approximately 7.4 (alkaline/neutral), methenamine remains stable and inactive in the systemic circulation. It is only when the drug is concentrated in the urine and the urine is acidified that formaldehyde is released. Formaldehyde is a non-specific denaturant of proteins and nucleic acids, providing the antiseptic effect. **Analysis of Options:** * **Option A (Incorrect):** Methenamine is well-absorbed orally but must be administered in enteric-coated tablets to prevent premature conversion to formaldehyde by gastric acid. * **Option B (Incorrect):** It is not significantly metabolized by the liver; it is excreted unchanged in the urine via glomerular filtration. * **Option D (Incorrect):** It is excreted primarily by glomerular filtration, not active tubular secretion. **Clinical Pearls for NEET-PG:** 1. **Urinary Acidifiers:** To ensure efficacy, methenamine is often co-administered with acidifying agents like **Vitamin C (Ascorbic acid)** or Ammonium chloride. 2. **Contraindication:** It is contraindicated in **Renal Insufficiency** because the mandelate or hippurate salts can cause systemic acidosis, and the drug cannot reach the bladder in therapeutic concentrations. 3. **Drug Interaction:** Do not co-administer with **Sulfonamides**; formaldehyde forms an insoluble precipitate with sulfonamides in the urine, increasing the risk of crystalluria. 4. **Spectrum:** It is a "universal" antiseptic; because formaldehyde's action is non-specific, resistance does not develop.

Question 905: What is the mechanism of action of chloramphenicol?

A. Preventing binding of tRNA to ribosome
B. Cell wall lysis
C. Cessation of peptide chain elongation (Correct Answer)
D. Inhibiting cell wall synthesis

Explanation: Chloramphenicol is a broad-spectrum, bacteriostatic antibiotic [1]. Its primary mechanism involves binding reversibly to the 50S ribosomal subunit at the peptidyl transferase site. By doing so, it inhibits the enzyme peptidyl transferase, which is responsible for forming peptide bonds between amino acids. Consequently, the addition of new amino acids to the growing polypeptide chain is blocked, leading to the cessation of peptide chain elongation. Analysis of Options: Option A (Incorrect): Preventing the binding of aminoacyl-tRNA to the A-site of the ribosome is the mechanism of Tetracyclines (which bind to the 30S subunit) [2]. Option B & D (Incorrect): Cell wall lysis and inhibition of cell wall synthesis are mechanisms characteristic of Bactericidal drugs like Beta-lactams (Penicillins, Cephalosporins) and Vancomycin. Chloramphenicol acts on protein synthesis, not the cell wall. High-Yield NEET-PG Pearls: 1. Resistance: Primarily mediated by the acquisition of a plasmid encoding chloramphenicol acetyltransferase, which inactivates the drug. 2. Gray Baby Syndrome: Occurs in neonates due to a deficiency of the hepatic enzyme glucuronyl transferase, leading to drug accumulation. 3. Bone Marrow Toxicity: Can cause dose-dependent suppression or idiosyncratic Aplastic Anemia (irreversible and fatal). 4. Spectrum: It is highly lipid-soluble and crosses the blood-brain barrier, making it historically significant for bacterial meningitis and enteric fever (though now limited by resistance and toxicity) [3].

Question 906: What is the drug of choice for acute pneumococcal lobar pneumonia?

A. Amoxicillin clavulanic acid combination
B. Ciprofloxacin
C. Co-trimoxazole
D. Crystalline penicillin (Penicillin G) (Correct Answer)

Explanation: **Explanation:** **1. Why Crystalline Penicillin (Penicillin G) is the Correct Answer:** *Streptococcus pneumoniae* (Pneumococcus) remains the most common cause of community-acquired lobar pneumonia. Despite rising resistance patterns globally, **Crystalline Penicillin (Penicillin G)** remains the traditional **drug of choice (DOC)** for sensitive strains in an inpatient setting. It is highly bactericidal against *S. pneumoniae* by inhibiting cell wall synthesis. For acute, severe cases requiring hospitalization, intravenous Penicillin G ensures rapid therapeutic concentrations in the lung parenchyma. **2. Why the Other Options are Incorrect:** * **Amoxicillin-Clavulanic Acid:** While effective for community-acquired pneumonia (CAP) and often used as empirical therapy (especially if *H. influenzae* is suspected), it is not the specific DOC for confirmed pneumococcal lobar pneumonia. Clavulanic acid adds no benefit against *S. pneumoniae* because its resistance mechanism involves altered Penicillin-Binding Proteins (PBPs), not beta-lactamase production. * **Ciprofloxacin:** This is a "second-generation" fluoroquinolone with poor activity against Gram-positive organisms like *S. pneumoniae*. "Respiratory quinolones" (e.g., Levofloxacin, Moxifloxacin) are used instead. * **Co-trimoxazole:** There is widespread resistance of *S. pneumoniae* to sulfonamides, making it an unreliable choice for acute lobar pneumonia. **3. Clinical Pearls for NEET-PG:** * **DOC for Outpatient Pneumococcal Pneumonia:** Oral Amoxicillin. * **Mechanism of Resistance:** *S. pneumoniae* develops resistance to penicillin via **alteration of PBPs**, not by producing beta-lactamase. * **Drug of Choice for Penicillin-resistant Pneumococci:** Vancomycin or Linezolid. * **Rust-colored sputum** is a classic clinical hallmark of pneumococcal pneumonia.

Question 907: Which of the following antibacterial agents causes both ototoxicity and nephrotoxicity?

A. Clindamycin
B. Azithromycin
C. Methicillin
D. Vancomycin (Correct Answer)

Explanation: **Explanation:** **Vancomycin** is a glycopeptide antibiotic primarily used for MRSA and other resistant Gram-positive infections [1]. Its side effect profile is a high-yield topic for NEET-PG. While modern, highly purified formulations have reduced the incidence, Vancomycin is classically associated with **nephrotoxicity** (acute kidney injury) and **ototoxicity** (tinnitus or hearing loss) [2]. These risks are significantly increased when Vancomycin is administered concurrently with other ototoxic/nephrotoxic drugs, such as Aminoglycosides or Loop diuretics [3]. **Analysis of Incorrect Options:** * **Clindamycin (A):** A lincosamide known for causing gastrointestinal distress and is the classic culprit for **Pseudomembranous colitis** (caused by *C. difficile* overgrowth). It does not cause ear or kidney damage. * **Azithromycin (B):** A macrolide antibiotic. While it can rarely cause transient hearing loss at very high doses, it is not nephrotoxic. Its primary side effects are GI upset and QT interval prolongation. * **Methicillin (C):** An older penicillin (now replaced by Oxacillin/Nafcillin). It is famously associated with **interstitial nephritis** (hypersensitivity reaction), but it is not ototoxic. **Clinical Pearls for NEET-PG:** * **Red Man Syndrome:** A common infusion-related reaction of Vancomycin caused by direct histamine release (not a true allergy). It is prevented by slowing the infusion rate. * **Therapeutic Drug Monitoring (TDM):** Essential for Vancomycin to maintain trough levels (typically 15–20 µg/mL) to minimize toxicity [3]. * **Mnemonic for Vancomycin Toxicity:** **"NOT"** – **N**ephrotoxicity, **O**totoxicity, **T**hrombophlebitis (and Red Man Syndrome).

Question 908: Which of the following protease inhibitors has an active metabolite?

A. Ritonavir
B. Indinavir
C. Nelfinavir (Correct Answer)
D. Saquinavir

Explanation: **Explanation:** **Nelfinavir** is unique among the HIV protease inhibitors (PIs) because it is metabolized primarily by the CYP2C19 isoenzyme into an **active metabolite** known as **M8 (hydroxy-t-butylamide)**. This metabolite possesses antiviral potency nearly equal to that of the parent drug, contributing significantly to its overall therapeutic efficacy. **Analysis of Options:** * **Nelfinavir (Correct):** It is the only PI listed that produces a major active metabolite. It is also notable for being one of the few PIs that does not strictly require "boosting" with ritonavir, though it is frequently associated with diarrhea as a side effect. * **Ritonavir:** While it is a potent inhibitor of CYP3A4 (used as a "pharmacokinetic booster"), its own metabolites are not clinically significant for antiviral activity. * **Indinavir:** Metabolized by the liver into inactive metabolites. It is clinically associated with nephrolithiasis (crystalluria) and requires high fluid intake. * **Saquinavir:** The first PI developed; it has low bioavailability and is metabolized into inactive compounds. **High-Yield NEET-PG Pearls:** * **Metabolic Side Effects:** All Protease Inhibitors are associated with **lipodystrophy** (buffalo hump), hyperlipidemia, and insulin resistance. * **Ritonavir Boosting:** Ritonavir is used at low doses to inhibit CYP3A4, thereby increasing the plasma concentration and half-life of other PIs (e.g., Lopinavir). * **Atazanavir:** Known for causing unconjugated hyperbilirubinemia and requires an acidic gastric pH for absorption (avoid PPIs). * **Darunavir:** Currently a preferred PI; contains a sulfonamide moiety (caution in sulfa allergy).

Question 909: All of the following drugs can cause hearing loss except?

A. Vancomycin
B. Kanamycin
C. Metronidazole (Correct Answer)
D. Quinine

Explanation: **Explanation:** The correct answer is **Metronidazole**. Ototoxicity (hearing loss and tinnitus) is a well-documented side effect of several classes of drugs, but Metronidazole is primarily associated with **neurotoxicity** (peripheral neuropathy, ataxia, and seizures) rather than auditory damage. **Why the other options are incorrect:** * **Kanamycin (Aminoglycosides):** These are the most notorious causes of ototoxicity. They cause irreversible damage to the hair cells in the cochlea (hearing loss) and the vestibular apparatus (balance issues) by generating reactive oxygen species. * **Vancomycin (Glycopeptides):** While less common than aminoglycosides, vancomycin can cause ototoxicity, especially when administered in high doses, in patients with renal impairment, or when used synergistically with other ototoxic drugs. * **Quinine (Antimalarials):** Quinine and its isomer quinidine cause a specific syndrome called **Cinchonism**, characterized by tinnitus, high-frequency hearing loss, dizziness, and visual disturbances. This is usually reversible upon discontinuation. **High-Yield Clinical Pearls for NEET-PG:** 1. **Ototoxic Drug Classes:** * **Aminoglycosides:** Neomycin (most toxic), Amikacin, Kanamycin, Gentamicin. * **Loop Diuretics:** Furosemide, Ethacrynic acid (highest risk). * **Chemotherapy:** Cisplatin, Carboplatin. * **Salicylates:** High-dose Aspirin (causes reversible tinnitus). 2. **Metronidazole Key Fact:** It is the drug of choice for anaerobic infections and pseudomembranous colitis. Its unique side effect is a **Disulfiram-like reaction** when consumed with alcohol. 3. **Monitoring:** Patients on long-term aminoglycosides or vancomycin should undergo periodic **audiometry** and renal function tests.

Question 910: All of the following are true about Praziquantel except?

A. It is the drug of choice for intestinal Taenia solium infection.
B. It is the drug of choice for schistosomiasis.
C. It increases the permeability for calcium ions and leads to flaccid paralysis of helminths. (Correct Answer)
D. Side effects include myalgia and arthralgia.

Explanation: ### Explanation **Praziquantel** is a broad-spectrum anthelmintic agent. The correct answer is **C** because it describes the wrong type of paralysis. **1. Mechanism of Action (Why Option C is the correct "Except"):** Praziquantel acts by increasing the cell membrane permeability to **calcium ions** in helminths. This influx of calcium causes massive depolarization, leading to **spastic (tetanic) paralysis**, not flaccid paralysis. At higher concentrations, it also causes vacuolization and disintegration of the schistosome tegument, allowing host immune cells to destroy the parasite. * *Note:* **Flaccid paralysis** is characteristic of drugs like **Piperazine** (GABA agonist) or **Ivermectin** (Glutamate-gated chloride channel opener). **2. Analysis of Other Options:** * **Option A:** Praziquantel is indeed the **Drug of Choice (DOC)** for adult tapeworm infections, including *Taenia solium* (pork tapeworm) and *Taenia saginata* (beef tapeworm). * **Option B:** It is the **DOC for all species of Schistosoma** (*S. haematobium, S. mansoni, S. japonicum*). * **Option D:** Common side effects include abdominal pain, dizziness, and headache. However, in patients with heavy parasite loads, the release of helminthic antigens can trigger systemic reactions like **myalgia, arthralgia**, and skin rashes. **3. High-Yield Clinical Pearls for NEET-PG:** * **DOC for Neurocysticercosis:** Albendazole is generally preferred over Praziquantel because it has better CNS penetration and its levels are increased by steroids (Dexamethasone). * **Contraindication:** Praziquantel is contraindicated in **Ocular Cysticercosis**, as the inflammatory response to dying larvae can cause irreversible eye damage. * **Drug Interaction:** Cimetidine increases Praziquantel levels, while Rifampicin and Phenytoin decrease them (via CYP450 induction).

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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