Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 1161: A 55-year-old patient with disseminated tuberculosis is prescribed antitubercular drugs. Which of the following adverse effects might be seen in a patient receiving regular intramuscular injections of streptomycin?

A. Blindness
B. Increased AST levels
C. Impotence
D. Deafness (Correct Answer)

Explanation: **Explanation:** Streptomycin is an **Aminoglycoside** antibiotic used as a first-line drug in the treatment of Tuberculosis (though its use is now limited due to the availability of oral alternatives). **1. Why Deafness is Correct:** Aminoglycosides are notorious for their **ototoxicity** and **nephrotoxicity**. Streptomycin specifically targets the 8th cranial nerve (Vestibulocochlear nerve). It accumulates in the endolymph and perilymph of the inner ear, leading to the destruction of sensory hair cells. While it is more commonly associated with **vestibular damage** (vertigo, ataxia), it also causes **cochlear damage**, resulting in permanent sensorineural hearing loss or deafness. **2. Why Incorrect Options are Wrong:** * **A. Blindness:** This is a classic side effect of **Ethambutol**, which causes optic neuritis (decreased visual acuity and red-green color blindness). * **B. Increased AST levels:** Hepatotoxicity (elevated liver enzymes) is a common side effect of **Isoniazid (INH), Rifampin, and Pyrazinamide**, but not Streptomycin. * **C. Impotence:** This is not a recognized side effect of any standard first-line antitubercular drugs. **Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Streptomycin inhibits bacterial protein synthesis by binding to the **30S ribosomal subunit**, causing mRNA misreading. * **Contraindication:** It is **teratogenic** (causes fetal ototoxicity) and is the only first-line ATT drug contraindicated in pregnancy. * **Excretion:** It is excreted unchanged by the kidneys; hence, dosage adjustment is vital in renal failure to prevent toxicity. * **Mnemonic for ATT Side Effects:** **E**thambutol (**E**ye), **S**treptomycin (**S**ound/Ears), **I**NH (**I**soniazid - **I**njures Nerves/Liver).

Question 1162: Which of the following antiviral drugs has dual antiviral activity against both HBV and HIV?

A. Saquinavir
B. Abacavir
C. Emtricitabine (Correct Answer)
D. None of the above

Explanation: **Explanation:** The correct answer is **Emtricitabine**. **Mechanism and Dual Activity:** Emtricitabine (FTC) is a Nucleoside Reverse Transcriptase Inhibitor (NRTI). It is a fluorinated analog of Lamivudine. Both Emtricitabine and Lamivudine inhibit the **Reverse Transcriptase** enzyme of HIV and the **DNA Polymerase** of the Hepatitis B Virus (HBV). This dual mechanism makes them essential components of Highly Active Antiretroviral Therapy (HAART) for patients co-infected with HIV and HBV. **Analysis of Incorrect Options:** * **A. Saquinavir:** This is a **Protease Inhibitor (PI)** used exclusively for HIV. It does not inhibit HBV DNA polymerase and thus lacks activity against HBV. * **B. Abacavir:** While this is an NRTI used for HIV, it is a guanosine analog that does **not** possess clinically significant activity against HBV. * **D. None of the above:** Incorrect, as Emtricitabine is a well-established dual-action agent. **High-Yield Clinical Pearls for NEET-PG:** * **Dual-Action Drugs (HIV + HBV):** The "LET" mnemonic is useful: **L**amivudine, **E**mtricitabine, and **T**enofovir (both TDF and TAF). * **Side Effects:** Emtricitabine is generally well-tolerated but can cause **hyperpigmentation** of the palms and soles, especially in children. * **Clinical Caution:** If a patient co-infected with HIV/HBV is treated with these drugs and then stops, they may experience a severe **"flare-up" of Hepatitis B**. * **Lamivudine vs. Emtricitabine:** Emtricitabine is more potent and has a longer half-life, allowing for once-daily dosing.

Question 1163: Which of the following drugs is NOT commonly used against enteric fever?

A. Amikacin (Correct Answer)
B. Ciprofloxacin
C. Ceftriaxone
D. Azithromycin

Explanation: **Explanation:** Enteric fever (Typhoid) is caused by *Salmonella typhi* and *S. paratyphi*, which are **intracellular** pathogens. Effective treatment requires drugs that can achieve high concentrations within cells and the biliary tract. **Why Amikacin is the correct answer:** Amikacin is an **Aminoglycoside**. Aminoglycosides are polar molecules that are poorly lipid-soluble; consequently, they have **poor intracellular penetration**. Furthermore, they are primarily effective against aerobic extracellular gram-negative bacilli and lack clinical efficacy against *Salmonella* species in the context of enteric fever. Therefore, they are not used in its management. **Why the other options are incorrect:** * **Ciprofloxacin (Fluoroquinolone):** Historically the drug of choice due to excellent intracellular penetration and high biliary concentration. However, its use is now limited in many regions due to rising Nalidixic acid resistance (NARST). * **Ceftriaxone (3rd Gen Cephalosporin):** Currently the **drug of choice** for empirical treatment of complicated or multidrug-resistant (MDR) enteric fever. It is administered parenterally. * **Azithromycin (Macrolide):** An excellent oral alternative for uncomplicated typhoid, especially in cases of fluoroquinolone resistance, due to its massive intracellular accumulation and long half-life. **High-Yield Clinical Pearls for NEET-PG:** 1. **Drug of Choice (Empirical):** Ceftriaxone. 2. **Best Oral Drug (for MDR strains):** Azithromycin. 3. **Carrier State:** The drug of choice to eradicate the chronic carrier state (in the gallbladder) is **Ciprofloxacin** (for 4–6 weeks) or Ampicillin. 4. **Historical Context:** Chloramphenicol was the first drug of choice but is now obsolete due to resistance and bone marrow toxicity.

Question 1164: An 82-year-old hospitalized patient with creatinine clearance of 25 mL/min has a microbial infection requiring treatment with antibiotics. Which one of the following drugs is least likely to require a dosage adjustment, either a smaller dose than usual or an increased interval between doses?

A. Amphotericin B
B. Erythromycin (Correct Answer)
C. Gentamicin
D. Imipenem-cilastatin

Explanation: The core concept tested here is the **route of elimination** of antimicrobial agents. In patients with renal impairment (CrCl < 25 mL/min), drugs primarily excreted by the kidneys require dose adjustments to prevent toxicity [2]. **Why Erythromycin is Correct:** Erythromycin is a macrolide antibiotic primarily metabolized by the liver and excreted via the **bile/feces**. Only a very small fraction (approx. 2–5%) is excreted unchanged in the urine. Therefore, its pharmacokinetics are not significantly altered by renal failure, making it the safest choice among the options for a patient with a low creatinine clearance without requiring dose modification [1]. **Analysis of Incorrect Options:** * **Gentamicin:** This is an aminoglycoside, which is highly polar and excreted **100% unchanged via glomerular filtration**. It is nephrotoxic and ototoxic; dosing must be strictly adjusted based on CrCl or plasma levels [3]. * **Imipenem-cilastatin:** This carbapenem is primarily eliminated by the kidneys. In renal failure, the drug (and its metabolite) can accumulate, significantly increasing the risk of **seizures**. * **Amphotericin B:** While notoriously nephrotoxic, it is actually not primarily cleared by the kidneys. However, in clinical practice, if a patient already has significant renal impairment (CrCl 25 mL/min), the use of conventional Amphotericin B is avoided or adjusted due to its high potential to further worsen renal function (azotemia). **NEET-PG High-Yield Pearls:** 1. **Drugs safe in Renal Failure (No dose adjustment):** Erythromycin, Azithromycin, Ceftriaxone, Doxycycline, Chloramphenicol, and Rifampicin [2]. 2. **Doxycycline** is the tetracycline of choice in renal failure because it is excreted via the gut (chelated with phosphates). 3. **Ceftriaxone** has dual excretion (biliary and renal), making it safe without adjustment in isolated renal or hepatic failure. 4. **Rule of Thumb:** Most "Aminoglycosides," "Penicillins," "Cephalosporins" (except Ceftriaxone), and "Fluoroquinolones" require dose reduction in renal impairment.

Question 1165: What is the radical cure for Plasmodium vivax?

A. Chloroquine
B. Tetracycline
C. Primaquine (Correct Answer)
D. Artesunate

Explanation: **Explanation:** The term **"Radical Cure"** in malaria refers to the elimination of both the erythrocytic stages (to treat symptoms) and the dormant liver stages (**hypnozoites**). *Plasmodium vivax* and *P. ovale* are unique because they form hypnozoites in the liver, which can cause relapses months or years later. **Why Primaquine is the Correct Answer:** Primaquine is a 8-aminoquinoline that acts as a **tissue schizonticide**. It is the only widely used drug effective against the latent hypnozoites of *P. vivax* and *P. ovale*. To achieve a radical cure, a standard course of Chloroquine (to kill blood stages) must be followed by Primaquine (usually for 14 days) to prevent relapse. **Analysis of Incorrect Options:** * **Chloroquine:** It is a potent blood schizonticide used for clinical cure (treating the acute attack) but has no activity against liver hypnozoites. * **Tetracycline:** Used as an adjunct (e.g., with Quinine) for resistant *P. falciparum*; it is too slow-acting and ineffective against hypnozoites. * **Artesunate:** An Artemisinin derivative used for rapid clearance of blood-stage parasites (especially in severe malaria). Like Chloroquine, it cannot eliminate hypnozoites. **High-Yield Clinical Pearls for NEET-PG:** 1. **G6PD Deficiency:** Before prescribing Primaquine, patients **must** be screened for G6PD deficiency, as the drug can cause life-threatening **acute hemolysis**. 2. **Tafenoquine:** A newer 8-aminoquinoline with a much longer half-life, allowing for a **single-dose** radical cure. 3. **Pregnancy:** Primaquine is **contraindicated** in pregnancy because the fetus is relatively G6PD deficient. 4. **Gametes:** Primaquine also has potent gametocidal activity against all species, including *P. falciparum*, helping prevent the spread of infection.

Question 1166: What is the most serious adverse effect of ketoconazole?

A. Adrenal insufficiency (Correct Answer)
B. Pellagra-like skin lesion
C. Liver injury
D. Prostate cancer

Explanation: **Explanation:** Ketoconazole is an imidazole antifungal that works by inhibiting the fungal enzyme 14-α-demethylase. However, its lack of selectivity leads to the inhibition of several human cytochrome P450 enzymes, most notably **11-β-hydroxylase** and **17,20-lyase**. **Why Adrenal Insufficiency is the correct answer:** Ketoconazole potently inhibits the steroidogenesis pathway in the adrenal cortex. By blocking 11-β-hydroxylase and 17-α-hydroxylase, it prevents the conversion of cholesterol into cortisol and aldosterone. This can lead to **primary adrenal insufficiency** (Addisonian-like crisis), which is considered the most serious systemic adverse effect. While this "side effect" is utilized therapeutically to treat Cushing’s syndrome, it remains a dangerous complication when the drug is used as an antifungal. **Analysis of Incorrect Options:** * **B. Pellagra-like skin lesion:** This is classically associated with **Isoniazid (INH)** therapy due to interference with Vitamin B3 (Niacin) metabolism, not ketoconazole. * **C. Liver injury:** While ketoconazole is hepatotoxic (elevated transaminases), adrenal suppression is generally considered more acutely life-threatening and a more unique pharmacological characteristic of this specific drug. * **D. Prostate cancer:** Ketoconazole is actually used as an **off-label treatment** for advanced prostate cancer because it inhibits androgen synthesis (via 17,20-lyase inhibition). It does not cause the disease. **High-Yield Clinical Pearls for NEET-PG:** * **Anti-androgenic effects:** Ketoconazole causes gynecomastia, loss of libido, and erectile dysfunction in males due to decreased testosterone. * **Drug Interactions:** It is a potent **CYP3A4 inhibitor**, leading to dangerous levels of drugs like Warfarin or Phenytoin. * **Absorption:** It requires an **acidic gastric pH** for absorption; avoid co-administration with H2 blockers or PPIs. * **Current Status:** Due to these toxicities, systemic ketoconazole has largely been replaced by safer triazoles (like Fluconazole) and is now primarily used topically.

Question 1167: Which of the following antitubercular drugs is associated with hypothyroidism?

A. Ethionamide (Correct Answer)
B. Streptomycin
C. Pyrazinamide
D. Rifampicin

Explanation: **Ethionamide** is a second-line antitubercular drug [2] (a thioamide derivative) that is structurally related to Methimazole and Carbimazole. Its association with **hypothyroidism** is due to its ability to inhibit the organic binding of iodine in the thyroid gland (the Wolff-Chaikoff effect), thereby interfering with the synthesis of thyroid hormones. This side effect is dose-dependent and reversible upon discontinuation. It is particularly common when Ethionamide is used in combination with Para-aminosalicylic acid (PAS), which also possesses antithyroid activity. **Analysis of Incorrect Options:** * **B. Streptomycin:** An aminoglycoside primarily known for **ototoxicity** (vestibular damage) and **nephrotoxicity** [1]. It does not affect thyroid function. * **C. Pyrazinamide:** Most commonly associated with **hepatotoxicity** and **hyperuricemia** (leading to gouty arthritis) due to inhibition of uric acid excretion. * **D. Rifampicin:** A potent **enzyme inducer** (CYP450). While it can accelerate the metabolism of levothyroxine in patients already on replacement therapy, it does not directly cause primary hypothyroidism. Its classic side effect is orange-red discoloration of body fluids. **High-Yield Clinical Pearls for NEET-PG:** * **Ethionamide & Prothionamide:** Both can cause hypothyroidism and significant GI intolerance [2] (metallic taste). * **PAS (Para-aminosalicylic acid):** Another ATT drug that causes hypothyroidism and goiter. * **Monitoring:** Patients on Ethionamide-containing regimens (especially in MDR-TB) should have their **TSH levels** monitored every 3–6 months. * **Visual Side Effects:** Ethionamide can also rarely cause optic neuritis, similar to Ethambutol.

Question 1168: Which protease inhibitor has a boosting effect?

A. Amprenavir
B. Tenofovir
C. Nelfinavir
D. Ritonavir (Correct Answer)

Explanation: **Explanation:** **Ritonavir** is the correct answer because it is a potent inhibitor of the **Cytochrome P450 3A4 (CYP3A4)** enzyme. In HIV therapy, it is primarily used at low doses as a **pharmacokinetic enhancer (booster)** rather than for its own antiviral activity. By inhibiting the metabolism of other Protease Inhibitors (PIs) like Lopinavir or Darunavir, Ritonavir increases their plasma concentration, prolongs their half-life, and allows for less frequent dosing and improved efficacy. This strategy is known as "Ritonavir-boosted therapy." **Analysis of Incorrect Options:** * **Amprenavir:** This is a protease inhibitor, but it does not possess the significant enzyme-inhibitory properties required to boost other drugs. It is actually the drug that often *requires* boosting. * **Tenofovir:** This is a Nucleotide Reverse Transcriptase Inhibitor (NRTI), not a protease inhibitor. It works by inhibiting the viral reverse transcriptase enzyme. * **Nelfinavir:** This is the only protease inhibitor that is **not** significantly boosted by Ritonavir because it is primarily metabolized by CYP2C19 rather than CYP3A4. **High-Yield Clinical Pearls for NEET-PG:** * **Cobicistat:** Another non-antiviral drug used solely as a booster (pharmacokinetic enhancer) in HAART regimens. * **Adverse Effects of PIs:** Look for "Buffalo Hump" (lipodystrophy), hyperglycemia (insulin resistance), and hyperlipidemia in clinical vignettes. * **Drug Interactions:** Due to CYP3A4 inhibition, Ritonavir has numerous drug-drug interactions (e.g., it can lead to toxicity of statins or benzodiazepines).

Question 1169: Isolated hyperbilirubinemia is classically seen as an adverse effect of which medication?

A. Isoniazid (INH)
B. Rifampin (Correct Answer)
C. Pyrazinamide
D. Capreomycin

Explanation: **Explanation:** **Correct Option: B (Rifampin)** Rifampin is known to cause **isolated hyperbilirubinemia** (predominantly unconjugated) because it competes with bilirubin for uptake into hepatocytes and interferes with its excretion into the bile. Unlike other hepatotoxic drugs, this effect is often transient and occurs without an elevation in serum transaminases (ALT/AST). It is a dose-dependent, pharmacological interference rather than true hepatocellular injury. **Analysis of Incorrect Options:** * **A. Isoniazid (INH):** While INH is hepatotoxic, it typically causes a **hepatocellular pattern** of injury characterized by significant elevations in ALT and AST. It does not cause isolated hyperbilirubinemia. * **C. Pyrazinamide:** This is the most hepatotoxic drug among the first-line ATT. It causes severe liver injury and can trigger gouty arthritis by inhibiting urate excretion, but it does not present with isolated bilirubin elevation. * **D. Capreomycin:** This is a second-line injectable agent (cyclic peptide). Its primary adverse effects are **nephrotoxicity** and **ototoxicity** (similar to aminoglycosides), not hepatotoxicity. **NEET-PG High-Yield Pearls:** * **Rifampin:** Potent inducer of Cytochrome P450 enzymes; causes orange-red discoloration of body fluids (urine, sweat, tears). * **Hepatotoxicity Profile:** Pyrazinamide > Isoniazid > Rifampin (P > I > R). * **Visual Side Effects:** Ethambutol causes optic neuritis (red-green color blindness). * **Clinical Rule:** If a patient on ATT develops jaundice with AST/ALT levels <3 times the upper limit of normal, Rifampin-induced competition for bilirubin uptake is the likely cause.

Question 1170: Which of the following is NOT considered a broad-spectrum antibiotic?

A. Tetracycline
B. Chloramphenicol
C. Doxycycline
D. Penicillin G (Correct Answer)

Explanation: **Explanation:** The classification of antibiotics is based on their **spectrum of activity**, which refers to the range of microorganisms they can effectively inhibit. **1. Why Penicillin G is the correct answer:** Penicillin G (Benzylpenicillin) is a **narrow-spectrum antibiotic**. It is primarily effective against Gram-positive cocci (e.g., *Streptococci*), some Gram-positive bacilli, and certain Gram-negative cocci (e.g., *Neisseria*). It is highly susceptible to inactivation by beta-lactamases and lacks significant activity against most Gram-negative rods. **2. Why the other options are incorrect:** * **Tetracycline & Doxycycline (Options A & C):** These belong to the Tetracycline class, which are classic **broad-spectrum** agents. They inhibit protein synthesis (30S subunit) and are effective against a wide array of Gram-positive and Gram-negative bacteria, as well as atypical organisms like *Rickettsia*, *Chlamydia*, and *Mycoplasma*. * **Chloramphenicol (Option B):** This is a potent **broad-spectrum** antibiotic that inhibits the 50S ribosomal subunit. It covers a vast range of aerobic and anaerobic bacteria, though its use is limited due to toxicity (e.g., Bone marrow suppression, Gray Baby Syndrome). **High-Yield NEET-PG Pearls:** * **Broad-spectrum definition:** Antibiotics that act on both Gram-positive and Gram-negative bacteria, often including anaerobes or atypicals. * **Extended-spectrum:** This term is often applied to semi-synthetic penicillins like **Amoxicillin** or **Piperacillin**, which have a broader range than Penicillin G but are narrower than Tetracyclines. * **Drug of Choice:** Penicillin G remains the drug of choice for **Syphilis** (*Treponema pallidum*). * **Resistance:** Most *Staphylococci* are now resistant to Penicillin G due to penicillinase production.

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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