Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 581: Isoniazid-induced peripheral neuritis is more common in which patient?

A. Slow acetylators (Correct Answer)
B. Fast acetylators
C. Slow oxidizers
D. Fast oxidizers

Explanation: **Explanation:**1. Why Slow Acetylators is the Correct Answer:Isoniazid (INH) is metabolized in the liver primarily by the enzyme **N-acetyltransferase 2 (NAT2)** via acetylation [1, 2]. In **slow acetylators**, the metabolism of INH is significantly delayed, leading to higher plasma concentrations of the drug [1].INH promotes the excretion of **Pyridoxine (Vitamin B6)** and inhibits the enzyme pyridoxine kinase, which converts B6 to its active form (pyridoxal phosphate) [1]. Higher levels of INH in slow acetylators result in a more profound deficiency of Vitamin B6. Since Vitamin B6 is essential for myelin synthesis and neurotransmitter metabolism, its deficiency manifests clinically as **peripheral neuritis** [1, 3].2. Why the Other Options are Incorrect:* **Fast Acetylators:** These individuals metabolize INH rapidly. While they are less prone to peripheral neuropathy, they are at a higher risk of **INH-induced hepatotoxicity** because they produce the toxic metabolite (acetylhydrazine) more quickly.* **Slow/Fast Oxidizers:** These terms refer to the Cytochrome P450 system (oxidation). While INH can inhibit certain CYP enzymes (like CYP2C19 and CYP3A4), its primary metabolic pathway is **acetylation**, not oxidation. Therefore, "oxidizer" status is not the determining factor for INH-induced neuropathy.3. NEET-PG High-Yield Pearls:* **Prophylaxis:** Peripheral neuritis can be prevented by co-administering **Pyridoxine (10–100 mg/day)** [1].* **Genetic Polymorphism:** Acetylator status is a classic example of pharmacogenetic variation [2].* **Other Side Effects:** INH is known for the triad of **Hepatotoxicity, Neurotoxicity, and Lupus-like syndrome** (the latter also more common in slow acetylators).* **Drug Interaction:** INH is a potent **enzyme inhibitor**, which can increase levels of drugs like Phenytoin and Carbamazepine.

Question 582: Cross-resistance between antimicrobial agents can be unidirectional or bidirectional. Unidirectional cross-resistance is observed in which of the following pairs?

A. Neomycin and streptomycin (Correct Answer)
B. Tetracycline and doxycycline
C. Sulphadiazine and sulphadoxine
D. None of the above

Explanation: ### Explanation **1. Why Neomycin and Streptomycin is Correct:** Cross-resistance occurs when resistance to one drug leads to resistance to another, usually within the same class. * **Unidirectional cross-resistance** means that resistance to drug 'A' confers resistance to drug 'B', but resistance to drug 'B' does not necessarily mean the organism is resistant to drug 'A'. * In the case of aminoglycosides, **Neomycin** is highly potent and susceptible to a wider array of bacterial inactivating enzymes compared to **Streptomycin**. Bacteria that develop resistance to Neomycin (often through multiple enzyme pathways) almost always show resistance to Streptomycin. However, bacteria resistant to Streptomycin (often via a single ribosomal mutation) frequently remain sensitive to Neomycin [1]. Therefore, the resistance flows in one direction. **2. Why Other Options are Incorrect:** * **B. Tetracycline and Doxycycline:** These exhibit **bidirectional (complete) cross-resistance**. The primary mechanism of resistance is the efflux pump (Tet-A) or ribosomal protection proteins, which generally affect all members of the tetracycline class simultaneously. * **C. Sulphadiazine and Sulphadoxine:** These are both Sulfonamides [3]. Resistance involves an alteration in the enzyme dihydropteroate synthase (DHPS). This mechanism provides **bidirectional cross-resistance** across the entire sulfonamide group [2]. **3. NEET-PG High-Yield Pearls:** * **Aminoglycosides:** Resistance is most commonly due to **R-factor mediated acquisition of inactivating enzymes** (adenylyltransferases, phosphoryltransferases, etc.). * **Gentamicin vs. Amikacin:** This is another classic example of unidirectional resistance. Amikacin is resistant to most aminoglycoside-inactivating enzymes; thus, Gentamicin-resistant strains are often sensitive to Amikacin, but Amikacin-resistant strains are almost always resistant to Gentamicin. * **Clinical Rule:** Always use the most "vulnerable" or narrow-spectrum drug first to preserve the efficacy of broader agents for later use.

Question 583: What is the drug of choice for secondary syphilis?

A. Chloramphenicol
B. Benzathine penicillin (Correct Answer)
C. Doxycycline
D. Vancomycin

Explanation: The drug of choice for all stages of syphilis (primary, secondary, and early latent) is **Benzathine Penicillin G**. Syphilis is caused by the spirochete *Treponema pallidum*, which remains highly sensitive to Penicillin. **Why Benzathine Penicillin is the Correct Choice:** *Treponema pallidum* has a very slow multiplication time (30–33 hours). To achieve a cure, the antibiotic concentration must be maintained above the minimum inhibitory concentration (MIC) for an extended period. Benzathine Penicillin is a **long-acting repository form** administered intramuscularly; a single dose of 2.4 million units provides sustained therapeutic blood levels for up to 3 weeks, effectively covering multiple replication cycles of the organism. **Analysis of Incorrect Options:** * **Chloramphenicol (A):** It is a bacteriostatic drug primarily used for enteric fever or meningitis in patients with severe allergies [2]; it has no role in the standard treatment of syphilis. * **Doxycycline (C):** While effective against *T. pallidum* [2], it is considered a **second-line alternative** only for non-pregnant patients with a penicillin allergy. It requires a 14-day course, leading to lower compliance compared to a single injection. * **Vancomycin (D):** This is a glycopeptide used for Gram-positive infections like MRSA [1]; it is not effective against spirochetes. **High-Yield Clinical Pearls for NEET-PG:** * **Jarisch-Herxheimer Reaction:** A common systemic reaction (fever, headache, myalgia) occurring within hours of the first penicillin dose due to the release of endotoxins from dying spirochetes. It is managed with aspirin/NSAIDs. * **Neurosyphilis:** Benzathine penicillin does not cross the blood-brain barrier. Therefore, **Aqueous Crystalline Penicillin G** (IV) is the drug of choice for neurosyphilis. * **Pregnancy:** Penicillin is the only recommended treatment. If a pregnant woman is allergic, she must undergo **desensitization** and then be treated with penicillin.

Question 584: Which of the following anti-tuberculosis (ATT) drugs can be used in patients with hepatic dysfunction?

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

Explanation: **Explanation:** The management of tuberculosis in patients with pre-existing liver disease requires a clear distinction between hepatotoxic and non-hepatotoxic drugs. **Why Streptomycin is the Correct Answer:** Streptomycin is an aminoglycoside that is primarily excreted unchanged by the **kidneys** via glomerular filtration. Unlike most first-line ATT drugs, it does not undergo hepatic metabolism and is **not hepatotoxic**. Therefore, it is the safest first-line agent to use in patients with hepatic dysfunction or chronic liver disease. **Analysis of Incorrect Options:** The remaining first-line ATT drugs are known for their potential to cause drug-induced liver injury (DILI): * **Pyrazinamide (C):** This is the **most hepatotoxic** drug among the first-line agents. It is strictly avoided or used with extreme caution in liver disease. * **Isoniazid (B):** It is metabolized in the liver via acetylation. It can cause both asymptomatic elevation of transaminases and severe hepatitis. * **Rifampicin (D):** It is a potent inducer of hepatic microsomal enzymes and can cause cholestatic jaundice. When combined with INH, the risk of hepatotoxicity increases synergistically. **NEET-PG High-Yield Pearls:** * **Hepatotoxicity Ranking:** Pyrazinamide > Isoniazid > Rifampicin (P > I > R). * **Safe Drugs in Liver Disease:** Streptomycin and Ethambutol are the two first-line drugs that are not hepatotoxic. * **Clinical Protocol:** In patients with unstable liver disease, the WHO-recommended regimen often consists of **"SHE"** (Streptomycin, Ethambutol, and a Quinolone) or avoiding PZA entirely. * **Monitoring:** ATT should be withheld if Serum Bilirubin > 2 mg/dL or if ALT/AST levels are > 3 times the upper limit of normal (with symptoms) or > 5 times (without symptoms).

Question 585: Which cephalosporin possesses anti-pseudomonal activity?

A. Cefazolin
B. Cefoperazone (Correct Answer)
C. Ceftriaxone
D. Cefuroxime

Explanation: **Explanation:**Cephalosporins are classified into generations based on their spectrum of activity [2]. The ability to cover *Pseudomonas aeruginosa* is a critical clinical distinction found primarily in specific **third and fourth-generation** agents.**Why Cefoperazone is correct:**Cefoperazone is a third-generation cephalosporin known for its potent anti-pseudomonal activity. Along with **Ceftazidime**, it is one of the two classic third-generation agents used to treat *Pseudomonas* infections [1]. A unique pharmacokinetic feature of Cefoperazone is its primary excretion through **bile** [2], making it useful in patients with renal failure but also increasing the risk of biliary sludge and a disulfiram-like reaction.**Why the other options are incorrect:*** **Cefazolin (Option A):** A first-generation cephalosporin [1], [2]. It has excellent Gram-positive coverage (especially *S. aureus*) but lacks the complexity to penetrate the outer membrane of *Pseudomonas* [1].* **Ceftriaxone (Option C):** While it is a third-generation cephalosporin with a broad Gram-negative spectrum, it notably **lacks activity** against *Pseudomonas* [1]. It is the drug of choice for meningitis and gonorrhea.* **Cefuroxime (Option B):** A second-generation cephalosporin [2]. It is effective against *H. influenzae* and *Moraxella*, but has no anti-pseudomonal efficacy [2].**High-Yield NEET-PG Pearls:**1. **Anti-pseudomonal Cephalosporins:** Ceftazidime (3rd gen), Cefoperazone (3rd gen), and Cefepime (4th gen) [1].2. **Biliary Excretion:** Cefoperazone and Ceftriaxone are the two cephalosporins that do not require dose adjustment in renal failure [2].3. **Side Effects:** Cefoperazone can cause hypoprothrombinemia (Vitamin K deficiency) and disulfiram-like reactions due to its methylthiotetrazole (MTT) side chain.

Question 586: What is the drug of choice for the exo-erythrocytic stage of malaria?

A. Chloroquine
B. Primaquine (Correct Answer)
C. Proguanil
D. Mefloquine

Explanation: The life cycle of the malaria parasite involves different stages, and drugs are classified based on which stage they target. The **exo-erythrocytic (EE) stage** refers to the phase where parasites (sporozoites) infect and multiply within the liver cells before entering the bloodstream.Why Primaquine is the Correct Answer:Primaquine is a tissue schizonticide. It is uniquely effective against the liver stages of all malaria species. Most importantly, it is the only drug capable of killing **hypnozoites** (the dormant liver stages) of *Plasmodium vivax* and *Plasmodium ovale* [1]. Therefore, it is the drug of choice for **radical cure** to prevent relapses.Analysis of Incorrect Options:* **Chloroquine:** A potent blood schizonticide [2]. It is highly effective against the erythrocytic (red blood cell) stage but has **no activity** against the exo-erythrocytic/liver stage.* **Proguanil:** While it has some causal prophylactic activity against the pre-erythrocytic stage, it is primarily used in combination (e.g., with Atovaquone) and is not the primary drug for eliminating established liver stages or hypnozoites.* **Mefloquine:** Like Chloroquine, this is a blood schizonticide used for prophylaxis and treatment of resistant malaria [1]. It does not eliminate the liver stages.High-Yield Clinical Pearls for NEET-PG:* **G6PD Deficiency:** Before prescribing Primaquine, patients **must** be screened for G6PD deficiency, as the drug can cause life-threatening acute hemolysis in these individuals.* **Pregnancy:** Primaquine is **contraindicated** in pregnancy because the G6PD status of the fetus cannot be determined.* **Tafenoquine:** A newer long-acting analog of Primaquine that also targets the EE stage and can be given as a single dose.* **Gametes:** Primaquine also has gametocidal action against all species, including *P. falciparum*, helping prevent the transmission of malaria [1].

Question 587: Which of the following is NOT an adverse effect of Amphotericin B?

A. Hepatotoxicity
B. Hyperkalemia (Correct Answer)
C. Thrombocytopenia
D. Arrhythmias

Explanation: **Explanation:** Amphotericin B is a potent antifungal that acts by binding to ergosterol in the fungal cell membrane, creating pores that lead to the leakage of intracellular contents [1]. However, it also binds to cholesterol in human cells, leading to significant systemic toxicity. **Why Hyperkalemia is the correct answer:** Amphotericin B is notoriously nephrotoxic [2]. It causes **renal tubular acidosis (Type 1)** and increases the permeability of the distal tubule. This leads to a profound wasting of electrolytes, specifically resulting in **Hypokalemia** (low potassium) and **Hypomagnesemia**, rather than hyperkalemia. Hyperkalemia is therefore not an adverse effect; in fact, potassium supplementation is often required during therapy. **Analysis of other options:** * **Hepatotoxicity:** While less common than nephrotoxicity, Amphotericin B can cause liver enzyme elevations and hepatic dysfunction. * **Thrombocytopenia:** Bone marrow suppression leading to anemia (most common), thrombocytopenia, and leukopenia can occur with prolonged use. * **Arrhythmias:** These can occur due to rapid infusion (infusion-related reaction) [2] or secondary to the severe electrolyte imbalances (hypokalemia/hypomagnesemia) caused by the drug. **NEET-PG High-Yield Pearls:** 1. **Nephrotoxicity** is the dose-limiting toxicity [2]. It can be minimized by "saline loading" (pre-infusion of 0.9% NaCl). 2. **Liposomal Amphotericin B** is preferred as it has reduced nephrotoxicity due to targeted delivery [1]. 3. **Infusion-related reactions** ("Shake and Bake"): Fever, chills, and rigors are common [2]. Pre-medication with NSAIDs, antihistamines, or hydrocortisone is often practiced. 4. **Anemia** caused by Amphotericin B is due to decreased erythropoietin production by the damaged kidneys.

Question 588: What is the drug of choice for treating infections caused by methicillin-resistant Staphylococcus aureus?

A. Macrolides
B. Third generation cephalosporins
C. Carbapenems
D. Glycopeptides (Correct Answer)

Explanation: **Explanation:** **1. Why Glycopeptides are correct:** Methicillin-resistant *Staphylococcus aureus* (MRSA) is resistant to almost all beta-lactam antibiotics due to an alteration in the target site—the **PBP-2a (Penicillin Binding Protein 2a)**, encoded by the *mecA* gene. This structural change prevents beta-lactams from binding to the cell wall. **Vancomycin**, a glycopeptide, remains the gold standard and drug of choice for MRSA. It works by binding to the **D-Ala-D-Ala** terminus of the nascent peptidoglycan chain, physically blocking cell wall synthesis. Since its mechanism does not involve PBPs, it bypasses the resistance mechanism of MRSA. **2. Why other options are incorrect:** * **Macrolides (e.g., Erythromycin):** These inhibit protein synthesis (50S subunit). However, MRSA strains frequently harbor *erm* genes that confer cross-resistance to macrolides, making them unreliable. * **Third-generation Cephalosporins (e.g., Ceftriaxone):** Like most beta-lactams, these cannot bind to the altered PBP-2a of MRSA. (Note: Ceftaroline, a 5th generation cephalosporin, is the only exception that covers MRSA). * **Carbapenems (e.g., Imipenem):** Despite being broad-spectrum, they are ineffective against MRSA for the same reason—lack of affinity for PBP-2a. **3. NEET-PG High-Yield Pearls:** * **Red Man Syndrome:** A common side effect of Vancomycin due to rapid histamine release (prevented by slow infusion). * **Alternative for MRSA:** If Vancomycin resistance (VRSA) or intolerance occurs, **Linezolid** (an Oxazolidinone) or **Daptomycin** (a Lipopeptide) are used. * **Daptomycin Caution:** It is **not** used for MRSA pneumonia because it is inactivated by pulmonary surfactant. * **DOC for MRSA Screening/Decolonization:** Topical **Mupirocin** applied to the nares.

Question 589: Which antifungal drug acts as an anti-metabolite?

A. Ketoconazole
B. Flucytosine (Correct Answer)
C. Terbinafine
D. Griseofulvin

Explanation: **Explanation:** **Flucytosine (5-FC)** is the correct answer because it is a pyrimidine antimetabolite. Its mechanism of action involves being taken up by fungal cells via the enzyme **cytosine permease**. Once inside, it is converted by **cytosine deaminase** into **5-fluorouracil (5-FU)**. This metabolite is further processed into 5-fluorodeoxyuridylic acid, which inhibits **thymidylate synthase**, thereby halting DNA synthesis. Since human cells lack cytosine deaminase, the drug exhibits selective toxicity. **Analysis of Incorrect Options:** * **Ketoconazole:** An imidazole derivative that inhibits the enzyme **14-α-demethylase**, preventing the conversion of lanosterol to ergosterol, thereby disrupting cell membrane synthesis. * **Terbinafine:** An allylamine that inhibits **squalene epoxidase**, leading to an accumulation of squalene (toxic) and a deficiency of ergosterol. * **Griseofulvin:** An antifungal that interferes with **microtubule function**, disrupting the mitotic spindle and inhibiting fungal mitosis. **NEET-PG High-Yield Pearls:** * **Synergy:** Flucytosine is rarely used alone due to rapid resistance development; it is most commonly combined with **Amphotericin B** for Cryptococcal meningitis. * **Side Effects:** The most significant dose-limiting toxicity is **bone marrow suppression** (anemia, leukopenia, thrombocytopenia) due to the conversion of 5-FC to 5-FU by intestinal flora. * **Spectrum:** It is primarily active against *Cryptococcus neoformans* and *Candida* species.

Question 590: Actinomycosis is sensitive to which of the following antimicrobial agents?

A. Streptomycin
B. Nystatin
C. Penicillin (Correct Answer)
D. Iodoxuridine

Explanation: **Explanation:** **Actinomycosis** is a chronic granulomatous infection caused by *Actinomyces israelii*, which are gram-positive, anaerobic, filamentous bacteria (not fungi, despite the name). 1. **Why Penicillin is Correct:** **Penicillin G** is the drug of choice for all forms of actinomycosis. Because the infection often presents with dense fibrosis and "sulfur granules," high doses are required over a prolonged period (weeks to months) to ensure adequate tissue penetration. For patients allergic to penicillin, tetracyclines or erythromycin are suitable alternatives. 2. **Why Other Options are Incorrect:** * **Streptomycin (Option A):** This is an aminoglycoside primarily used for aerobic gram-negative bacteria and *Mycobacterium tuberculosis*. It is ineffective against the anaerobic *Actinomyces*. * **Nystatin (Option B):** This is a polyene antifungal. Since *Actinomyces* are true bacteria (lacking ergosterol in their cell walls), antifungal agents have no effect on them. * **Iodoxuridine (Option C):** This is an antiviral agent (pyrimidine analog) used topically for Herpes Simplex Keratitis. It has no antibacterial properties. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** *Actinomyces* are often described as "branching filamentous bacteria." * **Diagnosis:** Look for **"Sulfur Granules"** in the discharge (yellowish clumps of organisms). * **Common Presentation:** "Lumpy Jaw" (Cervicofacial actinomycosis) following dental procedures or poor oral hygiene. * **Key Distinction:** Do not confuse *Actinomyces* (anaerobic, treated with Penicillin) with *Nocardia* (aerobic, acid-fast, treated with Sulfonamides/TMP-SMX). A common mnemonic is **SNAP**: **S**ulfa for **N**ocardia, **A**ctinomyces gets **P**enicillin.

Practice by Chapter

Beta-Lactam Antibiotics

Practice Questions

Aminoglycosides

Practice Questions

Macrolides and Ketolides

Practice Questions

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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