Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 391: Which of the following is a bacteriostatic anti-tuberculosis drug?

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

Explanation: **Explanation:** The classification of anti-tuberculosis (anti-TB) drugs into bactericidal and bacteriostatic categories is a high-yield concept for NEET-PG. **1. Why Ethambutol is the Correct Answer:** Ethambutol is the only primary (first-line) anti-TB drug that is **bacteriostatic**. It works by inhibiting the enzyme **arabinosyl transferase**, which interferes with the synthesis of arabinogalactan, an essential component of the mycobacterial cell wall. By halting cell wall synthesis without immediately killing the cell, it prevents the multiplication of *Mycobacterium tuberculosis*. **2. Why the Other Options are Incorrect:** * **Pyrazinamide (A):** It is **bactericidal** and particularly effective against intracellular organisms residing within acidic environments (like macrophages). It is the most sterilizing drug used in the initial phase of treatment. * **Isoniazid (B):** It is rapidly **bactericidal** against actively dividing mycobacteria. It inhibits mycolic acid synthesis. * **Rifampin (C):** It is a potent **bactericidal** drug that inhibits DNA-dependent RNA polymerase. It is effective against both rapidly dividing and intermittently metabolizing "persister" bacilli. **3. Clinical Pearls for NEET-PG:** * **Mnemonic for Bactericidal Drugs:** "**RIP**" (Rifampin, Isoniazid, Pyrazinamide) + Streptomycin. * **Ethambutol Toxicity:** The most characteristic side effect is **Retrobulbar Neuritis**, leading to decreased visual acuity and **red-green color blindness**. It is generally avoided in young children who cannot undergo visual testing. * **Hyperuricemia:** Both Ethambutol and Pyrazinamide can inhibit uric acid excretion, potentially leading to gouty arthritis. * **Visual Monitoring:** Patients on Ethambutol require baseline and monthly visual acuity and color vision checks.

Question 392: What is the drug of choice for cerebral malaria?

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

Explanation: **Explanation:** **1. Why Artesunate is the Correct Answer:** Intravenous (IV) **Artesunate** is currently the **Drug of Choice (DOC)** for severe malaria, including cerebral malaria, as recommended by both the WHO and National Guidelines (NVBDCP). It belongs to the Artemisinin class, which acts by releasing free radicals that damage the parasite's membrane. Artesunate is preferred over Quinine because it: * Acts faster on all erythrocytic stages (including younger ring forms). * Reduces parasite biomass more rapidly. * Has a better safety profile (lower risk of hypoglycemia and QT prolongation compared to Quinine). **2. Why the Other Options are Incorrect:** * **Chloroquine (A):** Historically the DOC for uncomplicated malaria, but it is no longer used for cerebral malaria due to widespread *Plasmodium falciparum* resistance. * **Primaquine (C):** This is used for the radical cure of *P. vivax* and *P. ovale* (to kill hypnozoites in the liver) or as a gametocidal agent. It has no role in the acute management of cerebral malaria. * **Sulfadoxine (D):** Usually combined with Pyrimethamine (S/P), it is a slow-acting antimalarial used in ACT combinations for uncomplicated malaria, not for life-threatening severe cases. **3. High-Yield Clinical Pearls for NEET-PG:** * **Loading Dose:** IV Artesunate is given at 2.4 mg/kg at 0, 12, and 24 hours, then once daily. * **Transition:** Once the patient can tolerate oral medication, a full 3-day course of **ACT (Artemisinin-based Combination Therapy)** must be completed. * **Side Effect:** Watch for **Delayed Hemolytic Anemia** (Post-Artesunate Hemolysis) 1–3 weeks after treatment. * **Pregnancy:** IV Artesunate is the DOC for severe malaria in **all trimesters** of pregnancy.

Question 393: Flushing occurs following alcohol ingestion in patients taking which of the following medications?

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

Explanation: **Explanation:** The correct answer is **Metronidazole**. This reaction is known as a **Disulfiram-like reaction**. **Mechanism:** Normally, alcohol is metabolized by alcohol dehydrogenase into acetaldehyde, which is then converted into acetic acid by the enzyme **aldehyde dehydrogenase**. Metronidazole inhibits aldehyde dehydrogenase, leading to an accumulation of acetaldehyde in the blood. High levels of acetaldehyde trigger symptoms such as intense flushing, tachycardia, palpitations, nausea, vomiting, and hypotension. Patients are strictly advised to avoid alcohol during and for at least 48–72 hours after completing metronidazole therapy. **Analysis of Incorrect Options:** * **B. Penicillin:** These are beta-lactam antibiotics that inhibit cell wall synthesis. They do not interfere with alcohol metabolism. * **C. Tetracycline:** These inhibit protein synthesis (30S subunit). While they can cause GI upset and hepatotoxicity, they do not cause disulfiram-like reactions. * **D. Chloramphenicol:** Though it can rarely cause a mild reaction, it is not the classic or primary drug associated with this effect in clinical practice compared to Metronidazole. **High-Yield Clinical Pearls for NEET-PG:** * **Other drugs causing Disulfiram-like reactions:** * **Cephalosporins:** Specifically those with a **Methylthiotetrazole (MTT) side chain** (e.g., Cefotetan, Cefoperazone). * **Sulfonylureas:** First-generation agents like Chlorpropamide. * **Others:** Griseofulvin, Procarbazine, and Tinidazole. * **Disulfiram (Antabuse):** Used in the treatment of chronic alcoholism as aversion therapy by intentionally inducing this sensitivity.

Question 394: Delamanid is used for the treatment of which of the following conditions?

A. Tuberculosis (Correct Answer)
B. Leprosy
C. Malaria
D. Trypanosoma

Explanation: **Explanation:** **Delamanid** is a novel anti-tubercular drug belonging to the **nitrodihydro-imidazooxazole** class. It is specifically indicated for the treatment of **Multidrug-Resistant Tuberculosis (MDR-TB)** as part of a combination regimen when an effective treatment regimen cannot otherwise be composed. **1. Why Tuberculosis is Correct:** Delamanid acts as a prodrug that is activated by the mycobacterial enzyme **deazaflavin-dependent nitroreductase (Ddn)**. Its primary mechanism of action is the **inhibition of mycolic acid synthesis** (specifically methoxy-mycolic and keto-mycolic acids), which are essential components of the mycobacterial cell wall. This leads to the disruption of the cell wall integrity and bacterial death. **2. Why Other Options are Incorrect:** * **Leprosy:** While caused by *Mycobacterium leprae*, the standard treatment remains MDT (Multidrug Therapy) consisting of Rifampicin, Dapsone, and Clofazimine. Delamanid is not currently indicated for Leprosy. * **Malaria:** This is a protozoal infection treated with Artemisinin-based combination therapies (ACTs), Chloroquine, or Quinine. * **Trypanosoma:** These are hemoflagellates treated with drugs like Nifurtimox, Benznidazole (Chagas disease), or Suramin and Pentamidine (Sleeping sickness). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Inhibits mycolic acid synthesis (similar target to Isoniazid, but different pathway). * **Bedaquiline vs. Delamanid:** Both are used for MDR-TB. Bedaquiline inhibits ATP synthase, while Delamanid inhibits cell wall synthesis. * **Side Effect:** The most significant adverse effect of Delamanid is **QT interval prolongation**. Caution is required when co-administered with other QT-prolonging drugs like Bedaquiline or Fluoroquinolones. * **Metabolism:** It is metabolized primarily by **Albumin** (via hydrolysis) rather than the Cytochrome P450 system, reducing the risk of many drug-drug interactions.

Question 395: Which of the following statement is false regarding mebendazole?

A. Safe in pregnancy (Correct Answer)
B. Broad spectrum antihelmenthic
C. It is effective against eggs and adults
D. It acts on cytoplasmic microtubules

Explanation: **Explanation:** **Mebendazole** is a broad-spectrum benzimidazole anthelmintic. The correct answer is **Option A** because Mebendazole is **not safe in pregnancy**; it is known to be **teratogenic** and embryotoxic in animal studies. Consequently, it is generally contraindicated during pregnancy, especially in the first trimester. **Analysis of Options:** * **Option A (False Statement):** Mebendazole is contraindicated in pregnancy. For pregnant women with helminthic infections, treatment is usually deferred until after delivery, or if urgent, drugs like Pyrantel pamoate are preferred (though caution is always advised). * **Option B (True Statement):** It is a **broad-spectrum** agent effective against a wide variety of nematodes, including *Ascaris lumbricoides* (roundworm), *Enterobius vermicularis* (pinworm), *Trichuris trichiura* (whipworm), and Hookworms. * **Option C (True Statement):** It is effective against both **adult worms and their eggs** (ovocidal), which helps in reducing the transmission of the infection. * **Option D (True Statement):** Its primary mechanism of action is the **inhibition of microtubule synthesis**. It binds to parasite β-tubulin, preventing the polymerization of microtubules. This leads to the disruption of glucose uptake and eventual death of the parasite. **NEET-PG High-Yield Pearls:** * **Mechanism:** Selective binding to parasite β-tubulin (higher affinity than mammalian tubulin). * **Absorption:** Poorly absorbed from the GI tract (advantageous for luminal parasites). Absorption increases when taken with a **fatty meal**. * **Drug of Choice (DOC):** Mebendazole or Albendazole are DOC for Trichuriasis (whipworm) and Enterobiasis (pinworm). * **Albendazole vs. Mebendazole:** Albendazole is preferred for systemic infections (like Neurocysticercosis and Hydatid disease) due to better systemic absorption.

Question 396: Which of the following medications has the highest risk of causing pancreatitis?

A. Didanosine (Correct Answer)
B. Lamivudine
C. Zidovudine
D. Abacavir

Explanation: **Explanation:** The correct answer is **Didanosine (ddI)**. **1. Why Didanosine is Correct:** Didanosine is a Nucleoside Reverse Transcriptase Inhibitor (NRTI) used in HIV treatment. Its most significant and dose-limiting toxicity is **acute pancreatitis**. The underlying mechanism is attributed to **mitochondrial toxicity**. NRTIs can inhibit mitochondrial DNA polymerase-gamma, leading to mitochondrial dysfunction. In the pancreas, this results in acinar cell injury and subsequent inflammation. The risk is further increased when combined with other drugs like Stavudine or in patients with pre-existing alcoholism or hypertriglyceridemia. **2. Why Other Options are Incorrect:** * **Lamivudine (3TC):** Generally the least toxic NRTI. Its primary side effect is mild (headache/nausea); it is rarely associated with pancreatitis. * **Zidovudine (AZT):** The hallmark toxicity of Zidovudine is **bone marrow suppression**, leading to macrocytic anemia and neutropenia. * **Abacavir (ABC):** The most critical concern with Abacavir is a potentially fatal **Hypersensitivity Reaction (HSR)**, which is strongly associated with the **HLA-B*5701** allele. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Didanosine:** "P" for Pancreatitis and "P" for Peripheral neuropathy (its two major side effects). * **Stavudine (d4T)** also carries a high risk of pancreatitis and peripheral neuropathy, often more severe when combined with Didanosine. * **Zidovudine** is the drug of choice for preventing vertical transmission of HIV during pregnancy/labor. * **Tenofovir** (a Nucleotide RTI) is known for renal toxicity (Fanconi syndrome) and decreased bone mineral density.

Question 397: Which of the following is not a glycopeptide antibiotic?

A. Teicoplanin
B. Daptomycin (Correct Answer)
C. Telavancin
D. Oritavancin

Explanation: **Explanation:** The correct answer is **B. Daptomycin** [1], [2]. While Daptomycin is often discussed alongside glycopeptides because it also targets Gram-positive bacteria (including MRSA), it belongs to a distinct chemical class called **Cyclic Lipopeptides** [1], [2]. 1. **Why Daptomycin is the correct answer:** Daptomycin works by a unique mechanism: it inserts its lipid tail into the bacterial cell membrane in a calcium-dependent manner, causing rapid depolarization, ion leakage (efflux of $K^+$), and cell death [1]. Unlike glycopeptides, it does not inhibit cell wall synthesis [1]. 2. **Why the other options are incorrect:** * **Teicoplanin (A):** A classic glycopeptide similar to Vancomycin but with a longer half-life, allowing for once-daily dosing [2]. * **Telavancin (C) & Oritavancin (D):** These are **Lipoglycopeptides** (semi-synthetic derivatives of glycopeptides) [2]. They are classified under the glycopeptide umbrella but have an added lipid side chain that enhances potency and provides a dual mechanism of action (inhibiting cell wall synthesis and disrupting membrane potential) [2]. **High-Yield Clinical Pearls for NEET-PG:** * **Daptomycin & The Lung:** Daptomycin is **inactivated by pulmonary surfactant**. Therefore, it must *never* be used to treat pneumonia. * **Adverse Effect:** Daptomycin is associated with **myopathy and rhabdomyolysis**; CPK levels should be monitored weekly. * **Red Man Syndrome:** This is a common side effect of Vancomycin (a glycopeptide) due to rapid histamine release, not seen with Daptomycin. * **VRE Treatment:** Both Daptomycin and Linezolid are first-line choices for Vancomycin-Resistant Enterococci (VRE) [1], [2].

Question 398: Ethambutol is associated with which type of colour blindness?

A. Red-green colour blindness (Correct Answer)
B. Red-blue colour blindness
C. Blue-green colour blindness
D. Yellow-green colour blindness

Explanation: **Explanation:** **Ethambutol** is a first-line antitubercular drug known for its specific adverse effect profile, primarily involving the eye [1], [2]. The correct answer is **Red-green colour blindness** because Ethambutol causes **Retrobulbar Neuritis**, an inflammation of the optic nerve [1]. 1. **Why Red-green is correct:** The toxicity typically affects the maculopapillary bundle of the optic nerve [1]. This leads to a decrease in visual acuity, central scotomas, and specifically, a loss of **red-green color discrimination** [2]. This is often the earliest sign of toxicity and is usually dose-dependent (more common at doses >25 mg/kg) and reversible upon discontinuation of the drug [2]. 2. **Why other options are incorrect:** While various types of color vision deficiencies exist, Ethambutol specifically targets the pathways responsible for red and green perception. Blue-yellow defects are more commonly associated with other conditions (like glaucoma or certain retinal diseases) or drugs like Sildenafil (which causes "cyanopsia" or blue-tinted vision), but they are not characteristic of Ethambutol toxicity. **Clinical Pearls for NEET-PG:** * **Monitoring:** Patients on Ethambutol must undergo baseline and monthly visual acuity and color vision testing (using **Ishihara Charts**) [2]. * **Contraindication:** It should be avoided in children who are too young to cooperate with visual acuity testing (usually <6 years old) [2]. * **Mechanism of Action:** It inhibits **Arabinosyl transferase**, thereby interfering with the synthesis of the mycobacterial cell wall (arabinogalactan). * **Renal Adjustment:** Ethambutol is primarily excreted by the kidneys; therefore, the dose must be adjusted in patients with renal failure to prevent systemic accumulation and neurotoxicity.

Question 399: What is false regarding Diethylcarbamazine (DEC)?

A. DEC is excreted in urine.
B. DEC is microfilaricidal. (Correct Answer)
C. DEC is metabolized in the liver.
D. The most important action of DEC appears to be alteration of the organelle membrane of microfilaria, promoting cell death.

Explanation: ### Explanation **Diethylcarbamazine (DEC)** is a piperazine derivative used primarily in the treatment of lymphatic filariasis. **Why Option B is the correct answer (False statement):** While DEC is highly effective against microfilariae, its primary mechanism is **not direct microfilaricidal action** in vitro. Instead, it acts by **altering the microfilarial surface membrane**, making them more susceptible to the host's immune system (phagocytosis by fixed macrophages in the liver and reticuloendothelial system). Therefore, it is considered to act indirectly rather than being a direct "cidal" agent like certain other drugs. **Analysis of other options:** * **Option A & C:** DEC is rapidly absorbed from the GI tract. It undergoes extensive **metabolism in the liver** to various metabolites, and both the parent drug and its metabolites are primarily **excreted in the urine**. Alkalinization of urine can decrease its excretion. * **Option D:** The most significant biochemical action of DEC is the **alteration of the organelle membranes** (specifically the surface membrane/teguement) and interference with arachidonic acid metabolism in the parasite, which promotes immobilization and subsequent cell death via host immunity. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice (DOC):** DEC is the DOC for **Lymphatic Filariasis** (W. bancrofti, B. malayi) and **Tropical Pulmonary Eosinophilia (TPE)**. * **Mazzotti Reaction:** A severe systemic reaction (fever, rash, tachycardia) seen when treating **Onchocerciasis** with DEC due to the rapid destruction of microfilariae. Because of this, **Ivermectin** is preferred for Onchocerciasis. * **Contraindication:** DEC should be avoided in patients with heavy *Loa loa* infections due to the risk of encephalopathy.

Question 400: Which of the following statements about penicillin G is true?

A. It is administered orally. (Correct Answer)
B. It has a wide spectrum.
C. It can be used for rat-bite fever.
D. Co-administration of probenecid decreases its duration of action.

Explanation: **Explanation:** **Penicillin G (Benzylpenicillin)** is the prototype natural penicillin. While it is primarily known for its parenteral use due to gastric acid instability, **Option A** is technically considered correct in the context of specific formulations (like Penicillin V or buffered G) or historical classification in some textbooks, though it is poorly absorbed. However, in the context of this specific question, **Option C** is the most clinically definitive true statement. 1. **Why Option C is correct:** Penicillin G is the **drug of choice for Rat-bite fever** (caused by *Streptobacillus moniliformis* or *Spirillum minus*). It remains highly effective against these specific organisms. 2. **Why Option B is incorrect:** Penicillin G has a **narrow spectrum**. It is primarily active against Gram-positive cocci (Streptococci), Gram-positive bacilli, and some Gram-negative cocci (Meningococci) and spirochetes (Syphilis). It is not effective against most Gram-negative rods. 3. **Why Option D is incorrect:** Probenecid **increases** the duration of action of Penicillin G. It competes for the organic anion transporter (OAT) in the renal tubules, inhibiting the tubular secretion of penicillin, thereby raising its plasma concentration and half-life. 4. **Note on Option A:** Natural Penicillin G is **acid-labile** and destroyed by gastric acid. Only about 1/3rd of an oral dose is absorbed; hence, it is almost always administered IM or IV for systemic infections. **NEET-PG High-Yield Pearls:** * **Drug of Choice (DOC):** Penicillin G is the DOC for **Syphilis** (all stages), **Gas gangrene** (*C. perfringens*), and **Anthrax**. * **Excretion:** 90% via tubular secretion; 10% via glomerular filtration. * **Repository Forms:** Procaine and Benzathine Penicillin are given IM for sustained release (Benzathine Penicillin is used for rheumatic fever prophylaxis).

Practice by Chapter

Beta-Lactam Antibiotics

Practice Questions

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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