Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 841: Which of the following should be monitored in a patient receiving linezolid therapy?

A. Renal function
B. Liver function
C. Auditory function
D. Platelet count (Correct Answer)

Explanation: **Explanation:** **Linezolid** is an oxazolidinone antibiotic primarily used for resistant Gram-positive infections like MRSA and VRE. The correct answer is **Platelet count** because the most significant dose-limiting toxicity of linezolid is **myelosuppression**, specifically **thrombocytopenia**. 1. **Why Platelet Count?** Linezolid inhibits mitochondrial protein synthesis in host cells when used for prolonged periods (usually >2 weeks). This leads to bone marrow suppression. Thrombocytopenia is the most common manifestation, though anemia and leukopenia can also occur. Therefore, a weekly Complete Blood Count (CBC) is mandatory for patients on long-term therapy. 2. **Why other options are incorrect:** * **Renal Function:** Unlike Vancomycin or Aminoglycosides, Linezolid does not require dose adjustment in renal failure as it is primarily metabolized non-renally. * **Liver Function:** While mild elevations in transaminases can occur, it is not the primary monitoring parameter compared to hematologic toxicity. * **Auditory Function:** Ototoxicity is a classic side effect of Aminoglycosides (e.g., Gentamicin) and Vancomycin, but not Linezolid. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Inhibits protein synthesis by binding to the **23S ribosomal RNA of the 50S subunit**, preventing the formation of the 70S initiation complex. * **MAO Inhibition:** Linezolid is a weak, reversible non-selective **MAO inhibitor**. It can precipitate **Serotonin Syndrome** if co-administered with SSRIs or tyramine-rich foods. * **Neuropathy:** Long-term use is associated with peripheral and **optic neuropathy** (which may be irreversible). * **Bioavailability:** It has **100% oral bioavailability**, allowing an easy switch from IV to oral dosing.

Question 842: Which of the following drugs is known to cause neuropsychiatric side effects?

A. Ethosuximide
B. Cycloserine (Correct Answer)
C. Pyrazinamide
D. Rifampicin

Explanation: **Explanation:** **Cycloserine** is a second-line antitubercular drug that acts by inhibiting D-alanyl-D-alanine synthetase and alanine racemase, thereby disrupting bacterial cell wall synthesis. It is highly lipid-soluble and readily crosses the blood-brain barrier. Its structural similarity to the amino acid D-alanine allows it to act as a partial agonist at the **NMDA (N-methyl-D-aspartate) receptors** in the CNS. This interaction leads to significant **neuropsychiatric side effects**, ranging from headache and somnolence to severe psychosis ("Cycloserine psychosis"), tremors, and convulsions. To mitigate these effects, it is often co-administered with Pyridoxine (Vitamin B6). **Analysis of Incorrect Options:** * **Ethosuximide:** An antiepileptic used for absence seizures. While it can cause drowsiness or GI upset, it is not primarily known for inducing the acute psychiatric disturbances associated with cycloserine. * **Pyrazinamide:** A first-line ATT drug primarily known for **hepatotoxicity** and **hyperuricemia** (leading to gout) due to inhibition of uric acid excretion. * **Rifampicin:** A bactericidal ATT drug known for causing **orange-colored secretions** (urine, sweat, tears) and being a potent **microsomal enzyme inducer**. **NEET-PG High-Yield Pearls:** * **Cycloserine Contraindication:** Strictly contraindicated in patients with a history of epilepsy or mental illness. * **Mnemonic:** Remember **"Psych-oserine"** to link Cycloserine with psychiatric side effects. * **Other drugs causing Psychosis:** Isoniazid (INH), Fluoroquinolones, and Corticosteroids.

Question 843: Which topical antifungal agent is used in the treatment of candidiasis?

A. Clotrimazole (Correct Answer)
B. Clotrimoxazole
C. Ornidazole
D. Propranolol

Explanation: ### Explanation **Correct Option: A. Clotrimazole** Clotrimazole is a broad-spectrum **imidazole** antifungal agent. It works by inhibiting the enzyme **lanosterol 14-alpha-demethylase**, which is essential for converting lanosterol to ergosterol (a key component of the fungal cell membrane). The resulting depletion of ergosterol leads to membrane instability and fungal cell death. It is highly effective against *Candida albicans* and is commonly used topically in the form of creams, lotions, or vaginal pessaries for oral, cutaneous, and vulvovaginal candidiasis. **Incorrect Options:** * **B. Clotrimoxazole:** This is a common distractor. It is an antibacterial combination of **Sulfamethoxazole and Trimethoprim** (also known as Co-trimoxazole). It is used for conditions like UTI, PCP pneumonia, and nocardiosis, but has no antifungal activity. * **C. Ornidazole:** This is a **nitroimidazole** derivative used as an antiprotozoal and anaerobic antibacterial agent. It is effective against *Entamoeba histolytica*, *Giardia*, and *Trichomonas*, but not against fungi. * **D. Propranolol:** This is a non-selective **beta-blocker** used for hypertension, migraines, and performance anxiety. It has no antimicrobial properties. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC):** For oral candidiasis (thrush) in infants, topical **Nystatin** is often preferred, while Clotrimazole is a standard alternative. * **Vaginal Candidiasis:** A single dose of oral **Fluconazole** (150 mg) is often the preferred systemic treatment, but topical Clotrimazole is the safest choice during **pregnancy**. * **Side Effects:** Topical imidazoles are generally well-tolerated but can cause local irritation or a burning sensation.

Question 844: Methicillin-resistant Staphylococcus aureus (MRSA) does NOT respond to which of the following drug classes?

A. Aminoglycosides
B. Lincosamides
C. Oxacillin (Correct Answer)
D. Vancomycin

Explanation: **Explanation:** **1. Why Oxacillin is the Correct Answer:** The defining characteristic of **Methicillin-resistant *Staphylococcus aureus* (MRSA)** is its resistance to almost all beta-lactam antibiotics (except 5th generation cephalosporins like Ceftaroline). This resistance is mediated by the **mecA gene**, which encodes an altered **Penicillin-Binding Protein (PBP2a)**. PBP2a has a very low affinity for beta-lactams. Since **Oxacillin** is a penicillinase-resistant penicillin (the clinical surrogate for Methicillin), MRSA is inherently resistant to it. In laboratory settings, Oxacillin resistance is the standard marker used to identify MRSA. **2. Analysis of Incorrect Options:** * **Aminoglycosides (e.g., Gentamicin):** While many MRSA strains show multi-drug resistance, aminoglycosides are not fundamentally excluded by the mecA mechanism. They may still be used in combination therapies for certain staphylococcal infections. * **Lincosamides (e.g., Clindamycin):** These are often used to treat community-acquired MRSA (CA-MRSA) skin and soft tissue infections, provided the strain does not possess inducible resistance (checked via the D-test). * **Vancomycin:** This is a glycopeptide and remains the **drug of choice** for serious systemic MRSA infections. It acts by binding to the D-Ala-D-Ala terminus of the peptidoglycan precursor, a mechanism unaffected by the PBP2a mutation. **Clinical Pearls for NEET-PG:** * **Drug of Choice for MRSA:** Vancomycin. * **Oral options for CA-MRSA:** Linezolid, Clindamycin, Cotrimoxazole, or Doxycycline. * **The "Exception" Beta-lactam:** Ceftaroline (5th gen Cephalosporin) is the only beta-lactam that covers MRSA. * **D-Test:** Used to detect inducible clindamycin resistance in staphylococci. * **VRSA Mechanism:** Resistance is due to the replacement of D-Ala-D-Ala with **D-Ala-D-Lac**.

Question 845: Raxiobacumab is used in the treatment of which condition?

A. Anthrax (Correct Answer)
B. Pontiac fever
C. Listeria infection
D. Clostridium infection

Explanation: **Explanation:** **Raxibacumab** is a recombinant human monoclonal antibody specifically designed for the treatment and prophylaxis of **inhalational anthrax** caused by *Bacillus anthracis*. **Mechanism of Action:** The pathogenicity of *B. anthracis* is primarily due to its toxins: Lethal Toxin (LT) and Edema Toxin (ET). Both toxins require a protein called **Protective Antigen (PA)** to bind to host cell receptors and facilitate entry. Raxibacumab works by binding directly to the PA, preventing it from interacting with host receptors. This neutralizes the toxin's effects even after the bacteria have been targeted by antibiotics. **Analysis of Options:** * **Option A (Anthrax):** Correct. Raxibacumab (and the newer agent Obiltoxaximab) are FDA-approved specifically for inhalational anthrax. * **Option B (Pontiac fever):** This is a mild, flu-like form of Legionellosis caused by *Legionella pneumophila*. It is self-limiting and does not require monoclonal antibody therapy. * **Option C (Listeria infection):** *Listeria monocytogenes* is typically treated with Ampicillin or Gentamicin. There is no role for toxin-neutralizing antibodies here. * **Option D (Clostridium infection):** While *C. difficile* is treated with Bezlotoxumab (another monoclonal antibody), Raxibacumab has no activity against Clostridial toxins. **High-Yield Clinical Pearls for NEET-PG:** * **Obiltoxaximab:** Another monoclonal antibody targeting the Protective Antigen (PA) of Anthrax. * **Standard Treatment:** Ciprofloxacin or Doxycycline are the first-line antibiotics for Anthrax. Raxibacumab is used as an **adjunct** to these antibiotics. * **Key Feature:** Raxibacumab is unique because it targets the **toxin**, not the bacteria itself, making it vital in advanced stages of the disease where toxin levels are high.

Question 846: Which is the most effective anti-tuberculosis drug acting on slow multiplying intracellular bacteria?

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

Explanation: **Explanation:** The effectiveness of anti-tuberculosis (ATT) drugs depends on their ability to target specific subpopulations of *Mycobacterium tuberculosis* based on their metabolic activity and location. **Why Pyrazinamide is Correct:** Pyrazinamide (PZA) is uniquely effective against **slowly multiplying intracellular bacilli** located within the acidic environment of macrophages (phagosomes). It is a prodrug converted by the bacterial enzyme *pyrazinamidase* into pyrazinoic acid. This acid accumulates in the acidic milieu, disrupting the bacterial membrane potential and inhibiting transport functions. Because it eliminates this "persistent" niche of bacteria, PZA is crucial for shortening the duration of TB treatment from 9–12 months to 6 months. **Why Other Options are Incorrect:** * **A. Isoniazid (INH):** It is the most potent **bactericidal** drug, but it acts primarily on **rapidly multiplying** extracellular bacilli. It is less effective against dormant or slowly dividing organisms. * **B. Ethambutol:** This is a **bacteriostatic** drug that inhibits cell wall synthesis (arabinosyl transferase). It is the weakest of the first-line drugs and does not specifically target intracellular populations. * **C. Streptomycin:** An aminoglycoside that is highly effective against **rapidly multiplying extracellular** bacilli (especially in cavities) but cannot penetrate cells effectively due to its polar nature; thus, it is inactive against intracellular bacteria. **NEET-PG High-Yield Pearls:** * **Sterilizing Activity:** Pyrazinamide and Rifampin have the highest sterilizing activity. * **Site of Action:** PZA = Acidic medium (Intracellular); Streptomycin = Alkaline medium (Extracellular). * **Side Effects:** PZA is the most **hepatotoxic** first-line drug and commonly causes **hyperuricemia** (leading to gout) by inhibiting renal uric acid excretion. * **Resistance:** Mutations in the **pncA gene** (encoding pyrazinamidase) lead to PZA resistance.

Question 847: Which of the following is a broad-spectrum antihelminthic?

A. Niclosamide
B. Albendazole (Correct Answer)
C. Mebendazole
D. Pyrantal pamoate

Explanation: **Explanation:** **Albendazole** is considered the drug of choice and a broad-spectrum antihelminthic because it is effective against a wide variety of nematodes (roundworms), cestodes (tapeworms), and even some trematodes. Its primary **mechanism of action** involves inhibiting microtubule synthesis by binding to parasite β-tubulin, which leads to glucose depletion and death of the parasite. Unlike Mebendazole, Albendazole has better systemic absorption (especially when taken with a fatty meal), allowing it to treat tissue-level infections like Neurocysticercosis and Hydatid disease. **Analysis of Incorrect Options:** * **Niclosamide (A):** It is a narrow-spectrum agent primarily used for intestinal tapeworms (Cestodes). It inhibits oxidative phosphorylation in the parasite but is ineffective against nematodes or tissue-stage infections. * **Mebendazole (C):** While also a benzimidazole, it has very poor systemic absorption. It is excellent for luminal (intestinal) nematodes like pinworm or whipworm but is less versatile than Albendazole for systemic or broad-spectrum use. * **Pyrantel Pamoate (D):** It is a depolarizing neuromuscular blocker effective only against luminal nematodes (Ascaris, Enterobius, and Hookworm). It is not effective against tapeworms or flukes. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC):** Albendazole is the DOC for **Hydatid disease** (*Echinococcus granulosus*) and **Neurocysticercosis** (*Taenia solium*). * **Absorption Tip:** Bioavailability of Albendazole increases significantly when administered with a **fatty meal**. * **Contraindication:** Most antihelminthics, including Albendazole, are generally avoided during the **first trimester of pregnancy** due to potential embryotoxicity. * **Praziquantel:** Often the DOC for most Trematodes (flukes) and Schistosomiasis.

Question 848: What is the drug of choice for treating infections caused by Taenia saginata and Taenia solium?

A. Metronidazole
B. Niclosamide
C. Praziquantel (Correct Answer)
D. Albendazole

Explanation: **Explanation:** The drug of choice for adult tapeworm infections (Cestodes) like *Taenia saginata* (beef tapeworm) and *Taenia solium* (pork tapeworm) is **Praziquantel**. **Mechanism of Action:** Praziquantel increases the permeability of the parasite's cell membrane to calcium ions. This induces a rapid influx of calcium, leading to severe muscular contraction and spastic paralysis of the worm. Additionally, it causes vacuolization and disintegration of the parasite’s tegument, allowing host immune cells to destroy it. **Analysis of Options:** * **Praziquantel (Correct):** It is highly effective against most cestodes and trematodes. A single oral dose is usually sufficient to clear intestinal taeniasis. * **Metronidazole:** This is an antiprotozoal and anaerobic antibacterial agent (used for *Giardia*, *Entamoeba*, and *Trichomonas*). It has no activity against helminths. * **Niclosamide:** Previously the drug of choice, it inhibits oxidative phosphorylation in the mitochondria of the worm. However, it is now considered a second-line alternative because it does not kill the eggs (increasing the theoretical risk of cysticercosis) and is less convenient than Praziquantel. * **Albendazole:** While Albendazole is the **drug of choice for Neurocysticercosis** (the larval form of *T. solium*) and Hydatid disease, it is less effective than Praziquantel for the primary treatment of adult intestinal tapeworms. **NEET-PG High-Yield Pearls:** 1. **DOC for H. nana:** Praziquantel. 2. **DOC for Neurocysticercosis:** Albendazole (often with corticosteroids to reduce inflammation). 3. **DOC for Hydatid Disease (*Echinococcus*):** Albendazole. 4. **Caution:** When treating *T. solium* with Praziquantel, ensure the patient does not have undiagnosed neurocysticercosis, as the drug can trigger an inflammatory response in the CNS.

Question 849: A 26-year-old African-American pilot develops anemia, hemoglobinemia, and hemoglobinuria several days after beginning primaquine chemoprophylaxis for Plasmodium vivax malaria. Special studies will likely reveal an abnormality in which of the following?

A. Duffy antigen
B. G6PD (Correct Answer)
C. Intrinsic factor
D. PIG-A

Explanation: **Explanation:** The clinical presentation of anemia, hemoglobinemia, and hemoglobinuria following the administration of **Primaquine** is a classic manifestation of **Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency**. **1. Why G6PD is correct:** G6PD is the rate-limiting enzyme in the Pentose Phosphate Pathway, responsible for generating **NADPH**. NADPH is essential for maintaining a pool of reduced **glutathione**, which protects red blood cells (RBCs) from oxidative stress. Primaquine is an oxidizing agent. In G6PD-deficient individuals (common in African-American and Mediterranean populations), the RBCs cannot neutralize the oxidative stress caused by the drug, leading to the denaturation of hemoglobin (**Heinz bodies**), damage to the RBC membrane (**Bite cells**), and subsequent intravascular hemolysis. **2. Why other options are incorrect:** * **Duffy antigen:** Absence of this antigen on RBCs provides resistance to *Plasmodium vivax* infection, as the parasite uses it as a receptor for entry. It is not related to drug-induced hemolysis. * **Intrinsic factor:** Deficiency leads to Vitamin B12 malabsorption and Megaloblastic (Pernicious) anemia, not acute hemolytic anemia. * **PIG-A:** Mutations in the PIG-A gene lead to **Paroxysmal Nocturnal Hemoglobinuria (PNH)** due to a deficiency of GPI-anchored proteins (CD55/CD59). While it causes hemoglobinuria, it is not triggered by Primaquine. **High-Yield NEET-PG Pearls:** * **Inheritance:** G6PD deficiency is an **X-linked recessive** disorder. * **Morphology:** Look for **Heinz bodies** (denatured Hb) and **Bite cells** (degluticytes) on peripheral smear. * **Contraindication:** Always screen for G6PD deficiency before prescribing Primaquine or Dapsone. * **Primaquine's Role:** It is the drug of choice for the **radical cure** of *P. vivax* and *P. ovale* as it acts on **hypnozoites** (latent liver stages).

Question 850: Pyridoxine should be administered when treating with which of the following drugs?

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

Explanation: **Explanation:** **Isoniazid (INH)** is the correct answer because it is a structural analogue of **Pyridoxine (Vitamin B6)**. INH interferes with the metabolism of B6 in two ways: it inhibits the enzyme *pyridoxine phosphokinase* (which converts B6 to its active form, pyridoxal phosphate) and increases the renal excretion of pyridoxine by forming isoniazid-pyridoxal hydrazones. A deficiency in pyridoxal phosphate leads to decreased synthesis of GABA (an inhibitory neurotransmitter), resulting in **peripheral neuropathy** (the most common side effect) and, in overdose, seizures. Administering prophylactic Pyridoxine (10–50 mg/day) prevents these neurological complications, especially in high-risk groups like diabetics, alcoholics, and pregnant women. **Why other options are incorrect:** * **Rifampicin:** Its primary side effects are hepatotoxicity and a harmless orange-red discoloration of body fluids (urine, sweat, tears). It does not affect B6 metabolism. * **Pyrazinamide:** Known for causing hyperuricemia (leading to gout) and hepatotoxicity. * **Streptomycin:** An aminoglycoside that causes ototoxicity (vestibular/cochlear damage) and nephrotoxicity. **High-Yield Clinical Pearls for NEET-PG:** * **INH Triad of Side Effects:** Peripheral neuropathy, Hepatotoxicity (most common), and Drug-induced Lupus (Anti-histone antibodies positive). * **Sideroblastic Anemia:** INH can also cause this because pyridoxal phosphate is a cofactor for ALA synthase, the rate-limiting enzyme in heme synthesis. * **Dose in Toxicity:** In acute INH poisoning (seizures), Pyridoxine is given intravenously in a gram-for-gram dose equal to the amount of INH ingested.

Practice by Chapter

Beta-Lactam Antibiotics

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Aminoglycosides

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Macrolides and Ketolides

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Tetracyclines

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Quinolones

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Sulfonamides and Trimethoprim

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Antimycobacterial Drugs

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Antifungal Agents

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Antiviral Drugs

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Antiparasitic Agents

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Principles of Antimicrobial Selection

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Antimicrobial Resistance

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