Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 411: Which of the following is NOT used in the management of AIDS?

A. Zidovudine
B. Stavudine
C. Ribavarin (Correct Answer)
D. Lamivudine

Explanation: **Explanation:** The management of AIDS involves **Highly Active Antiretroviral Therapy (HAART)**, which specifically targets the Human Immunodeficiency Virus (HIV), a retrovirus. **Why Ribavirin is the Correct Answer:** Ribavirin is a purine nucleoside analog, but it is **not** used for HIV/AIDS. Its primary clinical use is in the treatment of **Hepatitis C** (in combination with interferon or direct-acting antivirals) and **Respiratory Syncytial Virus (RSV)** in children. It works by interfering with viral RNA synthesis and capping. Notably, Ribavirin can actually antagonize the phosphorylation of Zidovudine and Stavudine, making it contraindicated or problematic in co-infected patients. **Analysis of Incorrect Options:** * **A. Zidovudine (AZT):** The first NRTI (Nucleoside Reverse Transcriptase Inhibitor) approved for HIV. It is a thymidine analog used to prevent mother-to-child transmission. * **B. Stavudine (d4T):** Another NRTI (thymidine analog) used in HIV management, though its use has declined due to mitochondrial toxicity (lipodystrophy and peripheral neuropathy). * **C. Lamivudine (3TC):** A potent NRTI (cytosine analog) used widely in HAART regimens. It is unique because it is active against both **HIV and Hepatitis B (HBV)**. **High-Yield Clinical Pearls for NEET-PG:** * **Zidovudine Side Effect:** Bone marrow suppression (Anemia/Neutropenia) is the dose-limiting toxicity. * **Lamivudine:** Least toxic NRTI; often used in the "TLD" (Tenofovir, Lamivudine, Dolutegravir) regimen. * **Ribavirin Toxicity:** Dose-dependent **hemolytic anemia** and significant **teratogenicity** (pregnancy must be avoided for 6 months post-treatment). * **Drug of Choice for RSV:** Ribavirin (Aerosolized).

Question 412: Which of the following antifungal drugs has only topical action?

A. Fluconazole
B. Ketoconazole
C. Itraconazole
D. Clotrimazole (Correct Answer)

Explanation: **Explanation:** The correct answer is **Clotrimazole**. **1. Why Clotrimazole is correct:** Clotrimazole is an imidazole antifungal that is used **exclusively for topical application**. When administered orally, it is poorly absorbed from the gastrointestinal tract and is associated with significant side effects (such as gastrointestinal distress and induction of hepatic enzymes). Therefore, its clinical use is restricted to topical formulations (creams, lotions, powders, and vaginal pessaries) for treating superficial fungal infections like dermatophytosis (tinea), cutaneous candidiasis, and vulvovaginal candidiasis. **2. Why the other options are incorrect:** * **Fluconazole (Option A):** A triazole with excellent oral bioavailability and CSF penetration. It is the drug of choice for systemic candidiasis and cryptococcal meningitis. * **Ketoconazole (Option B):** The first orally effective broad-spectrum azole. While it is used topically (e.g., in anti-dandruff shampoos), it is also available as an oral tablet, though its systemic use has declined due to hepatotoxicity and inhibition of steroid synthesis (anti-androgenic effects). * **Itraconazole (Option C):** A triazole used orally for systemic infections (histoplasmosis, blastomycosis) and onychomycosis. It is highly lipophilic and requires an acidic gastric environment for absorption. **3. High-Yield Facts for NEET-PG:** * **Topical-only Azoles:** Clotrimazole, Econazole, Miconazole, Oxiconazole, and Butoconazole. * **Mechanism of Action:** All azoles inhibit the enzyme **14-alpha-demethylase**, preventing the conversion of lanosterol to ergosterol, an essential component of the fungal cell membrane. * **Nystatin** is another antifungal that is used only topically (or "topically" within the GI tract via oral suspension) because it is too toxic for systemic (IV) use. * **Terbinafine** is the drug of choice for dermatophytoses (especially onychomycosis) and can be used both topically and orally.

Question 413: Which antitubercular drug is to be avoided in pregnancy?

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

Explanation: **Explanation** The correct answer is **Streptomycin**. **Why Streptomycin is avoided:** Streptomycin is an **aminoglycoside** and is strictly contraindicated in pregnancy (FDA Category D). It crosses the placental barrier and is known to be **ototoxic** to the developing fetus. Exposure during pregnancy can lead to permanent **congenital deafness** due to damage to the 8th cranial nerve (vestibulocochlear nerve) and potential nephrotoxicity. **Why other options are incorrect:** * **Isoniazid (INH):** Considered safe in pregnancy. However, it is usually co-administered with **Pyridoxine (Vitamin B6)** to prevent peripheral neuropathy in both the mother and the fetus. * **Rifampicin:** Considered safe and is a core component of the RNTCP/NTEP regimen for pregnant women. It has no proven teratogenic effects in humans. * **Ethambutol:** Considered the safest of the first-line antitubercular drugs during pregnancy. It does not cross the placenta in significant amounts to cause fetal harm. **High-Yield Clinical Pearls for NEET-PG:** 1. **Standard Regimen:** The WHO and NTEP recommend the same 6-month regimen (2HREZ + 4HR) for pregnant women as for non-pregnant adults, **excluding Streptomycin**. 2. **Pyrazinamide:** While some older guidelines were cautious, the WHO currently considers Pyrazinamide safe for use in pregnancy. 3. **Teratogenicity:** Streptomycin is the only first-line antitubercular drug with confirmed human teratogenicity (Ototoxicity). 4. **Breastfeeding:** All first-line anti-TB drugs are compatible with breastfeeding as they are excreted in breast milk in negligible concentrations.

Question 414: Which of the following drugs does NOT cause a curare-like effect?

A. Chloramphenicol (Correct Answer)
B. Polymyxin
C. Tetracycline
D. Streptomycin

Explanation: ### Explanation The "curare-like effect" refers to the **neuromuscular blockade (NMB)** caused by certain antibiotics, which can lead to muscle weakness or respiratory paralysis, especially when used alongside skeletal muscle relaxants or in patients with Myasthenia Gravis. **1. Why Chloramphenicol is the correct answer:** Chloramphenicol acts by inhibiting the 50S ribosomal subunit to prevent protein synthesis. Unlike several other classes of antibiotics, it has **no documented interference** with the release of acetylcholine or the sensitivity of the post-junctional membrane at the neuromuscular junction. Therefore, it does not cause neuromuscular blockade. **2. Why the other options are incorrect:** * **Streptomycin (Aminoglycosides):** This is the most common class associated with this effect. Aminoglycosides inhibit the pre-junctional release of Acetylcholine (ACh) and reduce post-junctional sensitivity to ACh. Neomycin and Streptomycin carry the highest risk. * **Polymyxin (B and Colistin):** These drugs exert a detergent-like action on the post-junctional membrane, causing non-depolarizing blockade that is often **not** reversible by neostigmine. * **Tetracycline:** While less common than aminoglycosides, tetracyclines can aggravate muscle weakness by chelating calcium ions, which are essential for ACh release at the motor endplate. ### NEET-PG High-Yield Pearls: * **Management:** Aminoglycoside-induced blockade is best reversed by **Intravenous Calcium Gluconate** (which promotes ACh release), while neostigmine is less consistently effective. * **Contraindication:** Avoid these "curare-like" antibiotics in patients with **Myasthenia Gravis**. * **Order of Potency for NMB:** Neomycin > Streptomycin > Amikacin > Gentamicin. * **Lincosamides (Clindamycin):** Also cause significant neuromuscular blockade by acting both pre- and post-junctionally.

Question 415: Acyclovir is indicated for which of the following conditions?

A. Enteric fever
B. Malaria
C. Herpes infection (Correct Answer)
D. Bacillary dysentery

Explanation: **Explanation:** **Correct Answer: C. Herpes infection** **Mechanism and Rationale:** Acyclovir is a potent **guanosine analog** that acts as a selective antiviral agent. Its mechanism of action is highly specific: it requires initial phosphorylation by the viral enzyme **thymidine kinase** to become acyclovir monophosphate. Host cell enzymes then convert it into acyclovir triphosphate, which inhibits viral DNA polymerase and causes **DNA chain termination**. Because it requires the viral enzyme for activation, it has low toxicity to healthy human cells. It is the drug of choice for **Herpes Simplex Virus (HSV-1, HSV-2)** and **Varicella-Zoster Virus (VZV)** infections. **Why other options are incorrect:** * **A. Enteric fever:** Caused by *Salmonella typhi* (bacteria). It is treated with fluoroquinolones (Ciprofloxacin) or third-generation cephalosporins (Ceftriaxone). * **B. Malaria:** Caused by *Plasmodium* species (protozoa). Treatment involves Artemisinin-based combination therapy (ACT), Chloroquine, or Quinine. * **D. Bacillary dysentery:** Caused by *Shigella* species (bacteria). It requires antibiotics like Ciprofloxacin or Azithromycin. **High-Yield NEET-PG Pearls:** * **Resistance:** Most commonly occurs due to the absence or mutation of viral **thymidine kinase**. * **Valacyclovir:** A prodrug of acyclovir with much higher oral bioavailability. * **Adverse Effects:** When given IV, it can cause **crystalline nephropathy** (prevented by adequate hydration). * **Drug of Choice:** Acyclovir is the gold standard for **Herpes Simplex Encephalitis**.

Question 416: Which of the following antitubercular drugs is safe in hepatitis?

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

Explanation: **Explanation:** The management of tuberculosis in patients with pre-existing liver disease requires careful selection of drugs, as most first-line agents are hepatotoxic. **Why Ethambutol is the correct answer:** Ethambutol is the only first-line antitubercular drug that is **not hepatotoxic**. It is primarily excreted through the kidneys (approx. 80%). Therefore, it does not require dose adjustment in hepatic impairment and is considered the safest option among first-line agents for patients with hepatitis or chronic liver disease. **Why the other options are incorrect:** * **Pyrazinamide (C):** This is the **most hepatotoxic** first-line drug. It is strictly contraindicated in patients with active liver disease. * **Isoniazid (A):** It is significantly hepatotoxic due to its metabolite, acetylhydrazine. It can cause a transient rise in transaminases or overt drug-induced hepatitis. * **Rifampicin (B):** It is hepatotoxic and can cause cholestatic jaundice. While less hepatotoxic than Pyrazinamide, it is a potent enzyme inducer and must be used with caution. **NEET-PG High-Yield Pearls:** * **Hepatotoxicity Ranking:** Pyrazinamide > Isoniazid > Rifampicin. * **Safe in Liver Disease:** Ethambutol and Streptomycin (though Streptomycin is second-line and injectable). * **Ethambutol Side Effect:** Its most characteristic side effect is **Optic Neuritis** (decreased visual acuity and red-green color blindness). It is contraindicated in children too young to undergo visual testing. * **Safe in Renal Failure:** Rifampicin and Isoniazid (primarily metabolized by the liver). Ethambutol requires dose adjustment in renal failure.

Question 417: What is the drug of choice for scabies?

A. Malathion
B. Ivermectin (Correct Answer)
C. Metronidazole
D. Benzoyl peroxide

Explanation: **Explanation:** The drug of choice for scabies has traditionally been topical **Permethrin (5%)** [1], [2]; however, in recent years and for competitive exams like NEET-PG, **Oral Ivermectin** is increasingly recognized as the systemic drug of choice, especially for institutional outbreaks, crusted (Norwegian) scabies, and cases where topical application is impractical. **Why Ivermectin is Correct:** Ivermectin is an anthelmintic that works by binding to glutamate-gated chloride channels in the invertebrate nerve and muscle cells, leading to paralysis and death of the *Sarcoptes scabiei* mite. It is highly effective, easy to administer (single oral dose of 200 μg/kg, repeated after 1–2 weeks), and ensures better compliance compared to messy topical creams. **Analysis of Incorrect Options:** * **A. Malathion:** An organophosphate used as a second-line treatment for head lice and scabies. It is effective but carries a risk of skin irritation and is flammable. * **C. Metronidazole:** An antiprotozoal and antibacterial agent (targeting anaerobes). It has no clinical efficacy against the scabies mite. * **D. Benzoyl Peroxide:** A keratolytic and antibacterial agent primarily used in the management of **Acne Vulgaris**; it has no role in treating parasitic infestations. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Topical:** Permethrin 5% (applied neck down, left for 8–12 hours) [2]. * **Crusted (Norwegian) Scabies:** Requires a combination of oral Ivermectin and topical Permethrin due to the high mite burden. * **Pregnancy/Lactation:** Permethrin is the safest choice (Category B) [1]. Ivermectin is generally avoided in children <15kg and pregnant women. * **Important Instruction:** Always treat all close contacts simultaneously to prevent re-infection ("Ping-pong" infection).

Question 418: The mechanism of action of chloramphenicol is inhibition of synthesis of:

A. Cell wall
B. Cell membrane
C. Protein (Correct Answer)
D. DNA

Explanation: **Explanation:** **Mechanism of Action (Correct Answer: C)** Chloramphenicol is a broad-spectrum bacteriostatic antibiotic. It inhibits **protein synthesis** by reversibly binding to the **50S ribosomal subunit**. Specifically, it inhibits the enzyme **peptidyl transferase**, which prevents the attachment of new amino acids to the growing peptide chain (transpeptidation). **Analysis of Incorrect Options:** * **A. Cell Wall:** Inhibition of cell wall synthesis is the mechanism of Beta-lactams (Penicillins, Cephalosporins), Vancomycin, and Bacitracin. * **B. Cell Membrane:** Agents like Polymyxins (Polymyxin B, Colistin) and Daptomycin act by disrupting the integrity of the bacterial cell membrane. * **D. DNA:** Inhibition of DNA synthesis or function is characteristic of Fluoroquinolones (DNA gyrase/Topoisomerase IV) and Metronidazole (DNA strand breakage). **High-Yield Clinical Pearls for NEET-PG:** * **Gray Baby Syndrome:** Occurs in neonates due to a deficiency of the hepatic enzyme **glucuronyl transferase**, leading to drug accumulation, cyanosis, and circulatory collapse. * **Bone Marrow Toxicity:** Can cause dose-dependent anemia or the more dreaded, idiosyncratic **Aplastic Anemia** (irreversible). * **Resistance:** Primarily mediated by the production of **Chloramphenicol acetyltransferase**, which inactivates the drug. * **Drug of Choice:** Historically used for Typhoid and Meningitis, but now largely restricted to topical use or life-threatening infections where safer alternatives are unavailable.

Question 419: Nevirapine is classified as which of the following?

A. Non-nucleoside reverse transcriptase inhibitor (NNRTI) (Correct Answer)
B. Nucleoside reverse transcriptase inhibitor (NRTI)
C. Integrase inhibitor
D. Neuraminidase inhibitor

Explanation: **Explanation:** **Nevirapine** is a potent and selective **Non-nucleoside Reverse Transcriptase Inhibitor (NNRTI)**. Unlike NRTIs, which are prodrugs that require intracellular phosphorylation, NNRTIs like Nevirapine are **non-competitive inhibitors** that bind directly to a specific hydrophobic pocket (the NNRTI-binding site) on the HIV-1 reverse transcriptase enzyme. This binding induces a conformational change that halts the conversion of viral RNA to DNA. **Analysis of Options:** * **Option B (NRTI):** Drugs like Zidovudine and Tenofovir are NRTIs. They are structural analogs of native nucleosides/nucleotides and act as competitive inhibitors that cause DNA chain termination. * **Option C (Integrase Inhibitors):** Drugs like Raltegravir and Dolutegravir inhibit the integrase enzyme, preventing the integration of viral DNA into the host genome. * **Option D (Neuraminidase Inhibitors):** Drugs like Oseltamivir and Zanamivir are used for Influenza, not HIV; they prevent the release of new virions from infected cells. **High-Yield Clinical Pearls for NEET-PG:** 1. **Metabolism:** Nevirapine is a potent **inducer of CYP3A4** enzymes, leading to significant drug-drug interactions. 2. **Adverse Effects:** The most characteristic side effects are **maculopapular rash** (which can progress to Stevens-Johnson Syndrome) and **hepatotoxicity**. 3. **Clinical Use:** Historically, a single dose of Nevirapine was used to prevent **Mother-to-Child Transmission (MTCT)** of HIV during labor, though it has largely been replaced by more robust regimens. 4. **Resistance:** A single mutation (K103N) can lead to high-level resistance across most first-generation NNRTIs.

Question 420: Why is intravenous quinine, used in the management of complicated and severe malaria, administered with 5% dextrose?

A. To avert the risk of hypoglycemia (Correct Answer)
B. To avert the risk of dehydration
C. To avert the risk of electrolyte imbalance
D. To maintain renal blood flow

Explanation: **Explanation:** **1. Why Option A is Correct:** Quinine is a potent stimulator of the **pancreatic beta cells**. It acts by blocking ATP-sensitive potassium channels, leading to membrane depolarization and the subsequent release of **insulin** (hyperinsulinemia). This drug-induced insulin secretion, combined with the high metabolic demands of the *Plasmodium falciparum* parasite and depleted glycogen stores in a severely ill patient, significantly increases the risk of **severe hypoglycemia**. Administering quinine in 5% dextrose acts as a continuous glucose source to counteract this effect. **2. Why Other Options are Incorrect:** * **Option B (Dehydration):** While fluid resuscitation is important in severe malaria, the specific choice of dextrose over normal saline is not primarily for volume replacement, but for metabolic stabilization. * **Option C (Electrolyte Imbalance):** Quinine does not significantly alter electrolytes directly. While it is a cinchona alkaloid that can affect cardiac conduction (QT prolongation), dextrose does not mitigate these electrolyte-related risks. * **Option D (Renal Blood Flow):** Maintaining renal perfusion is crucial in preventing Blackwater fever or Acute Tubular Necrosis in malaria, but this is achieved through titrated fluid therapy (isotonic crystalloids), not specifically by the glucose component. **High-Yield Clinical Pearls for NEET-PG:** * **Cinchonism:** The classic triad of quinine toxicity includes tinnitus, deafness, and visual disturbances. * **Drug of Choice:** While IV Artesunate is now the preferred first-line agent for severe malaria (WHO guidelines), Quinine remains a vital alternative. * **Blackwater Fever:** This is severe intravascular hemolysis and hemoglobinuria associated with quinine use in *P. falciparum* infection. * **Pregnancy:** Quinine-induced hypoglycemia is particularly common and severe in pregnant patients.

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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