Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 541: Ganciclovir is more effective than acyclovir against which virus?

A. Cytomegalovirus (CMV) (Correct Answer)
B. Influenza virus
C. Herpes simplex virus
D. None of the above

Explanation: **Explanation:** The correct answer is **Cytomegalovirus (CMV)**. **1. Why Ganciclovir is more effective against CMV:** While both Ganciclovir and Acyclovir are nucleoside analogs that inhibit viral DNA polymerase, Ganciclovir is specifically designed with a higher affinity for the enzymes found in CMV. * **Mechanism:** In CMV-infected cells, Ganciclovir undergoes its initial phosphorylation by a specific viral protein kinase called **UL97**. This results in much higher intracellular concentrations of the active triphosphate form within CMV-infected cells compared to Acyclovir. * **Acyclovir Limitation:** Acyclovir requires the viral enzyme **Thymidine Kinase (TK)** for activation. CMV lacks TK, making Acyclovir significantly less potent against it. **2. Analysis of Incorrect Options:** * **Influenza virus:** This is an RNA virus. Both Ganciclovir and Acyclovir target DNA polymerase; therefore, they have no activity against Influenza. Influenza is treated with neuraminidase inhibitors (e.g., Oseltamivir). * **Herpes simplex virus (HSV):** While Ganciclovir is active against HSV, **Acyclovir** remains the drug of choice due to its superior safety profile and lower toxicity. Ganciclovir is reserved for CMV because of its significant side effects. **3. High-Yield NEET-PG Pearls:** * **Drug of Choice:** Ganciclovir is the drug of choice for **CMV Retinitis** and CMV prophylaxis in transplant patients. * **Dose-Limiting Toxicity:** The most important side effect of Ganciclovir is **Bone Marrow Suppression** (specifically neutropenia and thrombocytopenia). * **Valganciclovir:** This is the L-valyl ester prodrug of ganciclovir with high oral bioavailability. * **Resistance:** Resistance to Ganciclovir in CMV occurs due to mutations in the **UL97 gene**. In such cases, **Foscarnet** or **Cidofovir** (which do not require viral phosphorylation) are used.

Question 542: All the following cephalosporins have significant activity against Pseudomonas Species except?

A. Cefoperazone
B. Ceftazidime
C. Cefotaxime (Correct Answer)
D. Cefepime

Explanation: **Explanation:** The core concept tested here is the spectrum of activity of different generations of Cephalosporins, specifically regarding **Pseudomonas aeruginosa**, a notorious Gram-negative pathogen. **Why Cefotaxime is the correct answer:** Cefotaxime is a **3rd generation cephalosporin**. While it has excellent activity against most Gram-negative bacteria (like *E. coli* and *Klebsiella*) and can cross the blood-brain barrier, it lacks significant activity against *Pseudomonas*. In the 3rd generation class, only a few specific drugs are "Anti-pseudomonal." **Analysis of Incorrect Options:** * **Ceftazidime (Option B):** This is a 3rd generation cephalosporin specifically designed for its potent activity against *Pseudomonas*. It is often considered the "gold standard" anti-pseudomonal cephalosporin. * **Cefoperazone (Option A):** Another 3rd generation cephalosporin with anti-pseudomonal activity. It is unique because it is primarily excreted via bile and does not require dose adjustment in renal failure. * **Cefepime (Option D):** This is a **4th generation cephalosporin**. 4th generation agents are characterized by an extended spectrum that covers both Gram-positive cocci and highly resistant Gram-negative organisms, including *Pseudomonas*. **High-Yield NEET-PG Pearls:** 1. **Anti-pseudomonal Cephalosporins:** Ceftazidime (3rd), Cefoperazone (3rd), Cefepime (4th), and Ceftobiprole (5th). 2. **Ceftriaxone vs. Cefotaxime:** Both are 3rd generation and neither covers *Pseudomonas*. 3. **Ceftaroline:** A 5th generation cephalosporin famous for **MRSA** coverage but, notably, it does **not** cover *Pseudomonas*. 4. **Mnemonic:** To remember anti-pseudomonal drugs, remember **"TAZ"** (Ceftazidime) and **"PIME"** (Cefepime).

Question 543: What is the drug of choice for the treatment of Trichomonas vaginalis?

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

Explanation: **Explanation:** **Correct Answer: D. Metronidazole** **Mechanism and Rationale:** Metronidazole is the drug of choice for *Trichomonas vaginalis*. It is a nitroimidazole prodrug that undergoes reductive activation by the enzyme **pyruvate:ferredoxin oxidoreductase (PFOR)** within anaerobic organisms. This process generates reactive nitro-radical intermediates that cause DNA strand breakage and inhibit protein synthesis, leading to cell death. While both Metronidazole and Tinidazole are effective, Metronidazole remains the standard first-line recommendation in most clinical guidelines. **Analysis of Incorrect Options:** * **A & C (Mebendazole and Albendazole):** These are **Benzimidazoles**, which act by inhibiting microtubule polymerization (binding to β-tubulin). They are anthelmintics used for intestinal nematodes (e.g., Ascaris, Hookworm) and have no activity against protozoa like *Trichomonas*. * **B (Tinidazole):** While Tinidazole is also highly effective against Trichomoniasis (often with fewer GI side effects and a longer half-life), Metronidazole is the traditional "textbook" drug of choice and the most frequently cited answer in standardized exams for this condition. **High-Yield Clinical Pearls for NEET-PG:** 1. **Disulfiram-like Reaction:** Patients must avoid alcohol during and for 48–72 hours after treatment with Metronidazole/Tinidazole due to inhibition of aldehyde dehydrogenase. 2. **Partner Treatment:** Trichomoniasis is a sexually transmitted infection (STI); **simultaneous treatment of the sexual partner** is mandatory to prevent "ping-pong" reinfection. 3. **Metallic Taste:** A common side effect of Metronidazole is a persistent metallic taste in the mouth. 4. **Pregnancy:** Metronidazole is considered safe for use in all trimesters of pregnancy for symptomatic Trichomoniasis.

Question 544: Which one of the following is not an antiretroviral drug?

A. Saquinavir
B. Ganciclovir (Correct Answer)
C. Indinavir
D. Atazanavir

Explanation: **Explanation:** The correct answer is **Ganciclovir** because it is an **anti-herpesvirus agent**, not an antiretroviral drug. **1. Why Ganciclovir is the correct answer:** Ganciclovir is a synthetic analogue of 2'-deoxyguanosine. Its primary mechanism involves the inhibition of viral DNA polymerase. It is specifically used for the treatment and prophylaxis of **Cytomegalovirus (CMV)** infections, particularly CMV retinitis in immunocompromised patients (e.g., those with AIDS). While it treats a complication of HIV, it does not target the HIV virus itself. **2. Analysis of Incorrect Options (Antiretrovirals):** * **Saquinavir, Indinavir, and Atazanavir** all belong to the **Protease Inhibitors (PIs)** class of antiretroviral therapy (ART). * **Mechanism:** They inhibit the viral protease enzyme (encoded by the *pol* gene), which is responsible for cleaving precursor polypeptides into functional proteins. This prevents the maturation of HIV particles, resulting in the production of immature, non-infectious virions. * **Suffix Clue:** Most Protease Inhibitors end with the suffix **"-navir"** (e.g., Ritonavir, Lopinavir, Darunavir). **3. NEET-PG High-Yield Pearls:** * **Ganciclovir Side Effect:** The dose-limiting toxicity is **bone marrow suppression** (neutropenia and thrombocytopenia). * **Atazanavir:** Known for causing unconjugated hyperbilirubinemia (benign jaundice) and is "lipid-friendly" compared to other PIs. * **Indinavir:** Associated with **nephrolithiasis** (kidney stones) due to crystalluria; patients must maintain high hydration. * **Protease Inhibitors** are generally associated with metabolic syndromes, including lipodystrophy (buffalo hump), insulin resistance, and dyslipidemia.

Question 545: All of the following are adverse effects of aminoglycosides except?

A. Nephrotoxicity
B. Ototoxicity
C. Neuromuscular blockade
D. Cholestatic jaundice (Correct Answer)

Explanation: **Explanation:** Aminoglycosides (e.g., Gentamicin, Amikacin, Streptomycin) are potent bactericidal antibiotics that inhibit protein synthesis by binding to the 30S ribosomal subunit. Their adverse effect profile is a high-yield topic for NEET-PG. **Why Cholestatic Jaundice is the Correct Answer:** Aminoglycosides are primarily excreted unchanged by the kidneys and do not undergo significant hepatic metabolism. Therefore, they are not associated with hepatotoxicity or **cholestatic jaundice**. Cholestatic jaundice is a classic adverse effect associated with **Macrolides** (specifically Erythromycin estolate) and certain penicillins (like Cloxacillin). **Analysis of Incorrect Options:** * **Nephrotoxicity (A):** Aminoglycosides accumulate in the renal proximal tubular cells, leading to acute tubular necrosis (ATN). This is usually reversible and manifests as a rise in serum creatinine. Neomycin is the most nephrotoxic. * **Ototoxicity (B):** They cause irreversible damage to the 8th cranial nerve. This can be **vestibular** (vertigo, ataxia—common with Streptomycin/Gentamicin) or **cochlear** (hearing loss—common with Amikacin/Kanamycin). * **Neuromuscular Blockade (C):** Aminoglycosides inhibit the pre-junctional release of Acetylcholine and decrease post-junctional sensitivity. This can lead to apnea, especially if used with skeletal muscle relaxants. It is contraindicated in **Myasthenia Gravis**. **High-Yield Clinical Pearls for NEET-PG:** 1. **Mnemonic (3 N’s):** **N**ephrotoxicity, **N**euromuscular blockade, and **N**inth (8th) nerve damage. 2. **Teratogenicity:** They can cause fetal ototoxicity (Category D). 3. **Treatment of Toxicity:** Calcium gluconate or Neostigmine can reverse the neuromuscular blockade. 4. **Monitoring:** Therapeutic Drug Monitoring (TDM) is recommended due to a narrow therapeutic index.

Question 546: Which of the following antimalarial drugs is gameticidal for all species of Plasmodium?

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

Explanation: **Explanation:** **Primaquine** is the correct answer because it is the only antimalarial drug that exhibits potent **gameticidal activity against all species of Plasmodium**, including *P. falciparum*. While many drugs can kill the gametocytes of *P. vivax* and *P. malariae*, *P. falciparum* gametocytes are notoriously resistant. Primaquine acts by generating reactive oxygen species (ROS) and interfering with the mitochondrial electron transport chain in the parasite, effectively preventing the transmission of malaria from humans to mosquitoes. **Analysis of Incorrect Options:** * **Quinine & Chloroquine:** These are primarily **blood schizonticides**. While they are gameticidal against *P. vivax* and *P. malariae*, they have **no effect** on the mature gametocytes of *P. falciparum*. * **Artesunate:** Artemisinin derivatives are potent blood schizonticides and do possess significant gameticidal activity against young/immature gametocytes. However, they are not as reliably effective against all stages of mature *P. falciparum* gametocytes as Primaquine. **High-Yield Clinical Pearls for NEET-PG:** 1. **Tissue Schizonticide:** Primaquine is also the drug of choice for the **radical cure** of *P. vivax* and *P. ovale* because it kills **hypnozoites** (latent liver stages), preventing relapse. 2. **G6PD Deficiency:** Before administering Primaquine, patients must be screened for G6PD deficiency. In these individuals, the drug causes oxidative stress leading to **acute hemolysis**. 3. **Pregnancy Contraindication:** Primaquine is contraindicated in pregnancy because the G6PD status of the fetus cannot be determined, risking fetal hemolysis. 4. **WHO Recommendation:** A single low dose of Primaquine (0.25 mg/kg) is recommended as a gameticide in *P. falciparum* malaria to reduce transmission in endemic areas.

Question 547: Which aminoglycoside is often combined with polymyxin and bacitracin for the treatment of topical infections?

A. Gentamicin
B. Tetracycline
C. Aztreonam
D. Neomycin (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Neomycin** **Neomycin** is a highly potent aminoglycoside that is too toxic for systemic use (causing severe nephrotoxicity and ototoxicity). However, it is excellently tolerated topically and is poorly absorbed through intact skin. It is the classic component of the **"Triple Antibiotic Ointment"** (Neosporin), which combines: 1. **Neomycin:** Targets aerobic Gram-negative bacteria. 2. **Polymyxin B:** Targets *Pseudomonas* and other Gram-negative organisms. 3. **Bacitracin:** Targets Gram-positive bacteria. This combination provides broad-spectrum coverage for minor cuts, wounds, and skin infections. **Analysis of Incorrect Options:** * **A. Gentamicin:** While used topically (e.g., for burns or impetigo), it is not the standard aminoglycoside paired with bacitracin and polymyxin in the classic triple-antibiotic formulation. It is more commonly used systemically for serious infections. * **B. Tetracycline:** This is a bacteriostatic protein synthesis inhibitor (30S) used for acne or ophthalmic infections, but it belongs to a different class and is not part of the standard polymyxin-bacitracin synergy. * **C. Aztreonam:** This is a Monobactam (cell wall inhibitor) used exclusively for Gram-negative infections (including *Pseudomonas*). It is administered parenterally and is not used in topical over-the-counter ointments. **High-Yield NEET-PG Pearls:** * **Mechanism of Action:** Aminoglycosides bind to the **30S ribosomal subunit**, causing mRNA misreading and inhibition of protein synthesis. They are **bactericidal**. * **Framycetin (Soframycin):** Another aminoglycoside used *only* topically (similar to Neomycin). * **Adverse Effect:** Neomycin is a common cause of **allergic contact dermatitis** (Type IV hypersensitivity). * **Oral Use:** Neomycin can be given orally for **hepatic encephalopathy** (to decrease ammonia-producing gut flora) and for **bowel preparation** before surgery because it is not absorbed systemically.

Question 548: Absorption of which of the following antimalarial drugs increases with food intake?

A. Mefloquine
B. Lumefantrine (Correct Answer)
C. Chloroquine
D. Amodiaquine

Explanation: **Explanation:** The correct answer is **Lumefantrine (Option B)**. **Why Lumefantrine is correct:** Lumefantrine is a highly lipophilic antimalarial drug. Its oral bioavailability is significantly enhanced (up to 16-fold) when administered with a **fatty meal**. This is clinically critical because Lumefantrine is almost always used in fixed-dose combinations with Artemether (ACT - Artemisinin-based Combination Therapy). Since Lumefantrine has a long half-life, its adequate absorption is essential to provide the "post-treatment cover" that prevents the recrudescence of *P. falciparum* malaria. **Why the other options are incorrect:** * **Mefloquine (Option A):** While food may slightly increase its absorption, it is not as clinically significant or characteristic as it is for Lumefantrine. Mefloquine is generally well-absorbed regardless of food intake. * **Chloroquine (Option C):** Chloroquine is rapidly and almost completely absorbed from the gastrointestinal tract. Its absorption is independent of food intake. * **Amodiaquine (Option D):** Similar to Chloroquine, Amodiaquine is well-absorbed orally, and its bioavailability is not significantly altered by food. **High-Yield Clinical Pearls for NEET-PG:** * **ACT Gold Standard:** Artemether + Lumefantrine is the most widely used ACT worldwide for uncomplicated *P. falciparum*. * **The "Fatty Meal" Rule:** Patients are specifically advised to take the dose with milk or fat-containing food to ensure therapeutic levels. * **Other drugs with increased absorption with food (Mnemonic: "Griseofulvin Loves Fat"):** Griseofulvin, Albendazole, Halofantrine, and Atovaquone also show significantly increased absorption with fatty meals. * **Lumefantrine Side Effect:** It can cause QT interval prolongation (though less severe than its predecessor, Halofantrine).

Question 549: Which of the following statements regarding penicillin is true?

A. It is acid-resistant.
B. It is excreted mainly through the kidney. (Correct Answer)
C. It is more concentrated in prostatic fluid.
D. It is a broad-spectrum antibiotic.

Explanation: **Explanation:** **Correct Option (B): It is excreted mainly through the kidney.** Penicillin G (Benzylpenicillin) is primarily eliminated by the kidneys (approx. 90% via **active tubular secretion** and 10% via glomerular filtration). Tubular secretion is mediated by the organic anion transporter (OAT). Because of this rapid clearance, penicillin has a short half-life (approx. 30 minutes). * **Clinical Correlation:** Probenecid can be co-administered to inhibit this tubular secretion, thereby increasing the plasma concentration and duration of action of penicillin. **Incorrect Options:** * **A. It is acid-resistant:** Natural Penicillin G is **acid-labile**, meaning it is destroyed by gastric acid and cannot be given orally. Only specific derivatives like Penicillin V (Phenoxymethylpenicillin) or Ampicillin are acid-stable. * **C. It is more concentrated in prostatic fluid:** Penicillins are lipid-insoluble and do not easily cross biological membranes. They achieve poor penetration into the prostate, eye, and CNS (unless the meninges are inflamed). * **D. It is a broad-spectrum antibiotic:** Penicillin G is a **narrow-spectrum** antibiotic, primarily effective against Gram-positive cocci (Streptococci), Gram-positive bacilli, and some Gram-negative cocci (Neisseria). **NEET-PG High-Yield Pearls:** 1. **Dose Adjustment:** Since it is renally excreted, dose reduction is mandatory in patients with renal failure to prevent neurotoxicity (seizures). 2. **Repository Forms:** Procaine and Benzathine penicillin are given IM to create a "depot" that bypasses the rapid renal clearance, providing sustained levels for days to weeks. 3. **Drug of Choice:** Penicillin G remains the drug of choice for **Syphilis** (*Treponema pallidum*).

Question 550: Which of the following drugs is used for prophylaxis of H1N1 influenza virus infections?

A. Oseltamivir (Correct Answer)
B. Abacavir
C. Tenofovir
D. Acyclovir

Explanation: **Explanation:** **Oseltamivir** is the correct answer because it is a **Neuraminidase Inhibitor** effective against both Influenza A (including H1N1) and Influenza B. Neuraminidase is an enzyme essential for the release of newly formed viral particles from infected host cells. By inhibiting this enzyme, Oseltamivir prevents the spread of the virus within the respiratory tract. It is FDA-approved for both the **treatment** (within 48 hours of symptom onset) and **chemoprophylaxis** of influenza. **Analysis of Incorrect Options:** * **Abacavir:** A Nucleoside Reverse Transcriptase Inhibitor (NRTI) used exclusively in the treatment of **HIV/AIDS**. It is notably associated with HLA-B*5701 hypersensitivity reactions. * **Tenofovir:** Another NRTI used for **HIV** and **Chronic Hepatitis B** infections. It has no activity against the influenza virus. * **Acyclovir:** A guanosine analogue that inhibits viral DNA polymerase. It is the drug of choice for **Herpes Simplex Virus (HSV)** and **Varicella-Zoster Virus (VZV)**, but it is ineffective against RNA viruses like Influenza. **High-Yield NEET-PG Pearls:** * **Route of Administration:** Oseltamivir is administered **orally**, whereas **Zanamivir** is administered via inhalation (contraindicated in asthma/COPD due to bronchospasm). * **Baloxavir Marboxil:** A newer single-dose drug for influenza that inhibits **cap-dependent endonuclease**. * **Amantadine/Rimantadine:** Older drugs that inhibit the **M2 ion channel**; they are no longer recommended for H1N1 due to widespread resistance. * **Prophylaxis Dose:** For Oseltamivir, the prophylactic dose is typically 75 mg once daily, while the therapeutic dose is 75 mg twice daily.

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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