Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 81: Which of the following antifungal agents binds to ergosterol?

A. Flucytosine
B. Terbinafine
C. Amphotericin B (Correct Answer)
D. Fluconazole

Explanation: **Explanation:** The correct answer is **Amphotericin B**. This drug belongs to the **Polyene** class of antifungals. Its mechanism of action involves binding directly to **ergosterol**, the primary sterol in fungal cell membranes. This binding creates transmembrane pores or channels, leading to the leakage of intracellular ions (like potassium) and small molecules, ultimately resulting in fungal cell death (fungicidal action). **Analysis of Incorrect Options:** * **Flucytosine (Option A):** This is an antimetabolite. It is converted into 5-fluorouracil (5-FU) inside the fungal cell, where it inhibits **thymidylate synthase**, thereby interfering with DNA and RNA synthesis. * **Terbinafine (Option B):** An Allylamine that inhibits the enzyme **Squalene epoxidase**. This leads to an accumulation of toxic squalene and a deficiency of ergosterol, but it does not bind to ergosterol itself. * **Fluconazole (Option D):** An Azole antifungal that inhibits the enzyme **14-alpha-demethylase** (a cytochrome P450 enzyme). This prevents the conversion of lanosterol to ergosterol. **High-Yield NEET-PG Pearls:** * **Amphotericin B** is the "Gold Standard" for most systemic fungal infections but is notorious for **nephrotoxicity** (azotemia, renal tubular acidosis, and hypokalemia). * **Nystatin** is another polyene that shares the same mechanism (binding to ergosterol) but is used only topically due to systemic toxicity. * **Selectivity:** Amphotericin B binds to ergosterol with much higher affinity than to human cholesterol, which accounts for its therapeutic index. * **Liposomal Amphotericin B** is preferred in clinical practice to reduce nephrotoxicity while maintaining efficacy.

Question 82: What is the drug of choice for the treatment of neurocysticercosis?

A. Pyrantel palmoate
B. Praziquantel
C. Albendazole (Correct Answer)
D. Ivermectin

Explanation: **Explanation:** **Neurocysticercosis (NCC)** is caused by the larval stage of *Taenia solium*. The drug of choice is **Albendazole** because of its superior pharmacokinetic profile in the Central Nervous System (CNS). 1. **Why Albendazole is Correct:** * **CNS Penetration:** Albendazole is a prodrug converted to albendazole sulfoxide, which achieves high concentrations in the cerebrospinal fluid (CSF) and brain parenchyma. * **Efficacy:** It is more effective than Praziquantel in killing viable cysts (especially subarachnoid and giant cysts) and typically requires a shorter duration of treatment (8–15 days). * **Mechanism:** It inhibits microtubule synthesis by binding to $\beta$-tubulin, leading to the death of the larvae. 2. **Why Other Options are Incorrect:** * **Praziquantel:** Once the drug of choice, it is now considered a second-line agent. It has lower CSF penetration, and its levels are significantly decreased when administered with corticosteroids (like Dexamethasone), which are routinely used in NCC to manage perilesional edema. * **Pyrantel Pamoate:** This is a depolarizing neuromuscular blocker used primarily for intraluminal intestinal parasites like *Ascaris* and Hookworms. It is not absorbed systemically and is ineffective against tissue cysts. * **Ivermectin:** While effective against Strongyloides and Onchocerciasis, it does not play a role in the standard management of NCC. **High-Yield Clinical Pearls for NEET-PG:** * **Steroid Co-administration:** Always administer corticosteroids (Dexamethasone) *before* or *with* Albendazole to prevent inflammatory neurological symptoms caused by the death of the cysticerci. * **Calcified Cysts:** If the cysts are already calcified (inactive), anthelmintic treatment is generally **not** required; management focuses on anti-epileptics. * **Ocular/Spinal NCC:** Anthelmintics are often contraindicated in ocular or spinal NCC due to the risk of irreversible inflammatory damage; surgery is preferred.

Question 83: Which of the following drugs is used for mass prophylaxis for the prevention of meningococcal meningitis?

A. Ciprofloxacin (Correct Answer)
B. Rifampicin
C. Ceftriaxone
D. Minocycline

Explanation: **Explanation:** The goal of prophylaxis in meningococcal meningitis is to eradicate the nasopharyngeal carriage of *Neisseria meningitidis*, thereby preventing the spread of the disease to close contacts and the community. **Why Ciprofloxacin is the Correct Answer:** **Ciprofloxacin** (500 mg, single oral dose) is currently the drug of choice for **mass prophylaxis**. It is highly effective in eliminating the carrier state, has the advantage of being a single-dose oral regimen, and is generally well-tolerated. In the context of public health and "mass" administration, its ease of use and cost-effectiveness make it superior to multi-dose regimens. **Analysis of Incorrect Options:** * **Rifampicin:** While historically the gold standard, it is no longer preferred for *mass* prophylaxis because it requires a 2-day course (four doses), induces cytochrome P450 enzymes (leading to drug interactions), and carries a high risk of developing rapid bacterial resistance. It also turns secretions orange, which can affect compliance. * **Ceftriaxone:** This is highly effective and is the drug of choice for prophylaxis in **pregnant women**. However, it must be administered via intramuscular injection, making it impractical for large-scale "mass" prophylaxis. * **Minocycline:** Although it can eradicate the carrier state, it is rarely used due to a high incidence of vestibular side effects (vertigo, dizziness, and nausea). **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (General/Mass):** Ciprofloxacin (Single dose). * **Drug of Choice (Pregnancy):** Ceftriaxone (Single IM injection). * **Alternative for Children:** Rifampicin or Ceftriaxone. * **Note:** Prophylaxis is indicated for close contacts (household, daycare) but is **not** recommended for casual contacts (school or office) unless an outbreak is confirmed.

Question 84: Resistance to acyclovir is most commonly due to mutation in a viral gene that encodes a protein that:

A. Converts viral RNA into DNA
B. Phosphorylates acyclovir (Correct Answer)
C. Transports acyclovir into the cell
D. Transports acyclovir out of the cell

Explanation: **Explanation:** **Mechanism of Action and Resistance:** Acyclovir is a guanosine analogue that acts as a "prodrug." To become active, it must undergo three stages of phosphorylation. The first and most critical step is the conversion of acyclovir to acyclovir monophosphate, which is mediated by the **viral enzyme Thymidine Kinase (TK)**. Host cell enzymes then complete the conversion to acyclovir triphosphate, which inhibits viral DNA polymerase. The most common mechanism of resistance in Herpes Simplex Virus (HSV) and Varicella-Zoster Virus (VZV) is a **mutation in the viral gene encoding Thymidine Kinase**, resulting in either deficient or altered TK activity. Without this initial phosphorylation, the drug remains inactive. **Analysis of Incorrect Options:** * **Option A:** This describes Reverse Transcriptase, which is relevant to HIV/Retroviruses, not the DNA-based Herpes viruses. * **Option C & D:** Acyclovir enters cells via passive diffusion or host transporters. Resistance is not typically mediated by transport proteins or efflux pumps in the viral context. **High-Yield Clinical Pearls for NEET-PG:** * **Most common resistance mechanism:** Loss of viral Thymidine Kinase activity (TK-deficient strains). * **Alternative mechanism:** Mutation in viral **DNA polymerase** (less common). * **Cross-resistance:** TK-deficient strains are also resistant to Valacyclovir, Famciclovir, and Ganciclovir. * **Drug of Choice for Acyclovir-resistant HSV:** **Foscarnet** or **Cidofovir** (neither requires phosphorylation by viral TK to be active). * **Side Effect:** Obstructive crystalline nephropathy (prevented by adequate hydration).

Question 85: Interstitial nephritis is most commonly associated with which of the following drugs?

A. Methicillin (Correct Answer)
B. Ampicillin
C. Amoxicillin
D. Cloxacillin

Explanation: **Explanation:** **1. Why Methicillin is the Correct Answer:** Acute Interstitial Nephritis (AIN) is a classic hypersensitivity reaction (Type IV) involving the renal interstitium. While many drugs can cause AIN, **Methicillin** is historically the most notorious culprit among the penicillins. It induces a cell-mediated immune response leading to inflammation, hematuria, and renal failure. Due to this high incidence of nephrotoxicity, Methicillin is no longer used clinically and has been replaced by safer alternatives like Cloxacillin and Nafcillin. **2. Why the Other Options are Incorrect:** * **Ampicillin & Amoxicillin:** These are aminopenicillins. While they can occasionally cause hypersensitivity reactions (like skin rashes or rare cases of AIN), their primary side effects are gastrointestinal (diarrhea) and non-specific allergic reactions. They are significantly less nephrotoxic than Methicillin. * **Cloxacillin:** This is a penicillinase-resistant penicillin (like Methicillin) used for Staphylococcal infections. It was specifically developed to provide the same spectrum as Methicillin but with a much lower risk of interstitial nephritis, making it the preferred clinical choice. **3. High-Yield Clinical Pearls for NEET-PG:** * **Classic Triad of AIN:** Fever, Rash, and Eosinophilia (though all three are present in only ~30% of patients). * **Urinary Finding:** Look for **eosinophiluria** (Hansel’s stain) and sterile pyuria. * **Other Drugs causing AIN:** NSAIDs, Sulfonamides, Rifampin, and Proton Pump Inhibitors (PPIs). * **MRSA:** The term "Methicillin-Resistant *Staphylococcus aureus*" persists in clinical nomenclature because Methicillin was the original drug used to test for resistance in this class.

Question 86: Which drugs are used in the treatment of Methicillin-resistant Staphylococcus aureus (MRSA)?

A. Quinupristin/dalfopristin (Correct Answer)
B. Atropine
C. Teicoplanin
D. Penicillin G

Explanation: **Explanation:** **MRSA (Methicillin-resistant Staphylococcus aureus)** is characterized by an altered Penicillin-Binding Protein (**PBP-2a**), encoded by the **mecA gene**, which renders almost all beta-lactam antibiotics ineffective. 1. **Why Quinupristin/Dalfopristin is correct:** This is a combination of streptogramins (B and A respectively) that acts synergistically to inhibit bacterial protein synthesis by binding to the 50S ribosomal subunit. It is specifically indicated for serious infections caused by multidrug-resistant Gram-positive bacteria, including **MRSA** and Vancomycin-resistant *Enterococcus faecium* (VRE). 2. **Why the other options are incorrect:** * **Atropine:** An anticholinergic drug used for bradycardia and organophosphate poisoning; it has no antimicrobial properties. * **Teicoplanin:** While Teicoplanin **is** effective against MRSA (it is a glycopeptide similar to Vancomycin), in the context of a single-choice question where Quinupristin/dalfopristin is marked as the primary correct answer, it serves as a distractor or indicates the question is looking for specific protein synthesis inhibitors. *Note: In many clinical exams, both would be correct, but Quinupristin/dalfopristin is a classic "reserve drug" answer.* * **Penicillin G:** MRSA is inherently resistant to all natural and antistaphylococcal penicillins due to the structural change in PBP. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice for MRSA:** Vancomycin remains the gold standard. * **Oral options for MRSA:** Linezolid, Clindamycin, and Cotrimoxazole. * **Newer agents:** Ceftaroline (the only 5th generation Cephalosporin active against MRSA), Daptomycin (cannot be used in pneumonia as it is inactivated by pulmonary surfactant), and Tigecycline. * **Side effect of Quinupristin/Dalfopristin:** Arthralgia and myalgia are common limiting factors.

Question 87: The drug used to treat acyclovir-resistant Herpes Simplex Virus (HSV) and Varicella Zoster Virus (VZV) infection is:

A. Foscarnet (Correct Answer)
B. Valacyclovir
C. Abacavir
D. Ganciclovir

Explanation: **Explanation:** **1. Why Foscarnet is the correct answer:** The primary mechanism of resistance to Acyclovir in HSV and VZV is the **deficiency or mutation of the viral enzyme Thymidine Kinase (TK)**. Acyclovir is a prodrug that requires initial phosphorylation by viral TK to become active. **Foscarnet** is a pyrophosphate analogue that directly inhibits viral DNA polymerase without requiring activation by phosphorylation. Therefore, it remains effective even when the virus lacks the TK enzyme, making it the drug of choice for acyclovir-resistant strains. **2. Why the other options are incorrect:** * **Valacyclovir:** This is an L-valyl ester prodrug of acyclovir. Since it is converted to acyclovir in the body, it still requires viral Thymidine Kinase for activation and will show cross-resistance with acyclovir. * **Abacavir:** This is a Nucleoside Reverse Transcriptase Inhibitor (NRTI) used in the treatment of HIV, not HSV or VZV. * **Ganciclovir:** While used for CMV, ganciclovir also requires initial phosphorylation (by viral TK in HSV or UL97 kinase in CMV). Strains resistant to acyclovir due to TK deficiency are often cross-resistant to ganciclovir. **3. High-Yield Clinical Pearls for NEET-PG:** * **Cidofovir:** Another drug used for acyclovir-resistant HSV; it is a nucleotide analogue that requires host cellular kinases, not viral TK, for activation. * **Foscarnet Toxicity:** The most important side effect is **Nephrotoxicity** (minimized by saline pre-loading) and **electrolyte imbalances** (hypocalcemia, hypomagnesemia, and hypokalemia). * **Drug of Choice for CMV Retinitis:** Ganciclovir (first-line); Foscarnet is used if ganciclovir fails.

Question 88: Which of the following statements about silver sulfadiazine is not true?

A. Used in burns dressings
B. Has antibacterial action against Pseudomonas
C. Possesses immunomodulatory action (Correct Answer)
D. A potential side effect is a hypersensitivity reaction

Explanation: ### Explanation **Silver Sulfadiazine** is a topical sulfonamide widely used in the management of burn wounds. **Why Option C is the correct answer:** Silver sulfadiazine does **not** possess immunomodulatory action. Its primary mechanism of action is purely antimicrobial. It acts by slowly releasing silver ions, which are toxic to bacteria (by binding to DNA and cell proteins), combined with the bacteriostatic effect of the sulfadiazine moiety (inhibiting folic acid synthesis). It is used for its local antiseptic properties rather than altering the immune response. **Analysis of Incorrect Options:** * **Option A:** It is the "gold standard" topical agent used in **burn dressings** to prevent and treat wound sepsis. * **Option B:** It has a broad spectrum of activity, including significant efficacy against **Pseudomonas aeruginosa**, a common and dangerous pathogen in burn patients. * **Option D:** Since it contains a sulfonamide, it can cause **hypersensitivity reactions** (rashes, itching) and, in rare cases, systemic absorption can lead to leukopenia or kernicterus in neonates. **High-Yield Clinical Pearls for NEET-PG:** * **Mafenide Acetate:** Another topical agent for burns; unlike silver sulfadiazine, it can penetrate eschar but may cause **metabolic acidosis** (due to carbonic anhydrase inhibition). * **Silver Nitrate:** Can cause electrolyte imbalances (hyponatremia) and staining of the skin/dressings. * **Contraindication:** Silver sulfadiazine should be avoided in pregnancy, newborns (risk of kernicterus), and patients with known sulfonamide allergies. * **Key Advantage:** It is painless upon application, unlike Mafenide which causes a stinging sensation.

Question 89: Which of the following drugs is contraindicated in G-6PD deficiency?

A. Primaquine (Correct Answer)
B. Pyrimethamine
C. Chloroquine
D. Artemether

Explanation: **Explanation:** **Primaquine** is the correct answer because it is a potent oxidizing agent. In patients with **Glucose-6-Phosphate Dehydrogenase (G-6PD) deficiency**, the red blood cells (RBCs) cannot generate sufficient NADPH to maintain a pool of reduced glutathione. Without reduced glutathione, the RBCs are unable to neutralize oxidative stress caused by Primaquine, leading to the denaturation of hemoglobin (forming **Heinz bodies**) and subsequent **acute hemolytic anemia**. **Analysis of Incorrect Options:** * **Pyrimethamine:** An antifolate (DHFR inhibitor) used in toxoplasmosis and malaria; it does not cause significant oxidative stress and is safe in G-6PD deficiency. * **Chloroquine:** A 4-aminoquinoline used for erythrocytic schizonts. While it can rarely cause hemolysis in severe G-6PD variants, it is generally considered safe at standard doses. * **Artemether:** An artemisinin derivative used for multidrug-resistant malaria. It acts via free radical production within the parasite but does not trigger hemolysis in G-6PD-deficient patients. **Clinical Pearls for NEET-PG:** * **Primaquine’s Role:** It is the drug of choice for the **radical cure** of *P. vivax* and *P. ovale* (targets hypnozoites) and acts as a **gametocide** for all species, including *P. falciparum*. * **Mandatory Screening:** Always screen for G-6PD levels before initiating Primaquine or **Tafenoquine** (a newer long-acting analog). * **Other High-Yield G-6PD Contraindications:** Sulfonamides, Dapsone, Nitrofurantoin, Nalidixic acid, and Fava beans. * **Peripheral Smear Finding:** Look for **"Bite cells"** (degmacytes) and **Heinz bodies** in the context of drug-induced hemolysis.

Question 90: What is the dose of Oseltamivir (Tamiflu) for chemoprophylaxis against H1N1 Influenza?

A. 75mg once daily for 5 days
B. 75 mg once daily for 7 days
C. 75 mg once daily for 10 days (Correct Answer)
D. 75 mg once daily for 6 weeks

Explanation: Oseltamivir is a **Neuraminidase Inhibitor** effective against both Influenza A (including H1N1) and Influenza B. The drug works by preventing the release of new virions from infected host cells, thereby limiting the spread of the virus within the respiratory tract. **1. Why Option C is correct:** For **post-exposure chemoprophylaxis** (after contact with a confirmed case), the standard adult dose is **75 mg once daily for 10 days** [2]. This duration ensures coverage during the typical incubation period of the virus, preventing the establishment of a clinical infection. **2. Why the other options are incorrect:** * **Option A (75 mg OD for 5 days):** This is not a standard regimen for prophylaxis. * **Option B (75 mg OD for 7 days):** While 7 days is used for some viral exposures, the WHO and CDC guidelines specifically mandate 10 days for H1N1 post-exposure prophylaxis. * **Option D (75 mg OD for 6 weeks):** This extended duration is reserved for **seasonal/pre-exposure prophylaxis** during a community outbreak or for immunocompromised patients who cannot be vaccinated [1]. **3. High-Yield Clinical Pearls for NEET-PG:** * **Treatment Dose:** For active H1N1 infection, the dose is **75 mg twice daily (BD) for 5 days** [1], [2]. * **Mechanism of Action:** Inhibits Neuraminidase, preventing the cleavage of sialic acid residues and trapping the virus within the cell. * **Timing:** For maximum efficacy, treatment should ideally start within **48 hours** of symptom onset. * **Pregnancy:** Oseltamivir is the drug of choice for H1N1 in pregnant women (Category C, but benefits outweigh risks). * **Alternative:** **Zanamivir** is an inhaled neuraminidase inhibitor but is contraindicated in patients with asthma/COPD due to the risk of bronchospasm [1].

Practice by Chapter

Beta-Lactam Antibiotics

Practice Questions

Aminoglycosides

Practice Questions

Macrolides and Ketolides

Practice Questions

Tetracyclines

Practice Questions

Quinolones

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Sulfonamides and Trimethoprim

Practice Questions

Antimycobacterial Drugs

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Antifungal Agents

Practice Questions

Antiviral Drugs

Practice Questions

Antiparasitic Agents

Practice Questions

Principles of Antimicrobial Selection

Practice Questions

Antimicrobial Resistance

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