Antimicrobial Agents — MCQs
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Metrifonate is effective against which of the following conditions?
Which of the following is used for single dose radical cure of Plasmodium vivax malaria?
Tetracycline is contraindicated in:
Which of the following cephalosporins can cause thrombocytopenia?
The primary reason for the use of drug combination in the treatment of tuberculosis is to:
Which of the following antimicrobials can be used for the treatment of diabetic gastroparesis?
Which of the following antimicrobial agents is exclusively used topically?
Linezolid belongs to which class of drugs?
Mebendazole is more effective than albendazole in the treatment of which of the following helminthic infections?
A patient with multidrug-resistant tuberculosis is undergoing treatment with anti-tubercular drugs. After a few months, the patient develops red-green color blindness. Which drug is most likely causing this adverse effect?
Antimicrobial Agents Indian Medical PG Practice Questions and MCQs
Question 131: Metrifonate is effective against which of the following conditions?
- A. Amoebiasis
- B. Leishmaniasis
- C. Schistosomiasis (Correct Answer)
- D. Giardiasis
Explanation: **Explanation:** **Metrifonate** (also known as Trichlorfon) is an organophosphate compound that acts as a prodrug. It is non-enzymatically converted into **dichlorvos**, a potent acetylcholinesterase inhibitor. In the context of helminthic infections, it inhibits the cholinesterase enzyme of the parasite, leading to paralysis and the subsequent detachment of the flukes from the pelvic venous plexus. **1. Why Schistosomiasis is Correct:** Metrifonate is specifically effective against ***Schistosoma haematobium*** (the urinary blood fluke). The paralyzed worms are swept from the bladder veins into the lungs, where they are trapped and destroyed by the host's immune system. While it is highly effective for *S. haematobium*, it is not effective against *S. mansoni* or *S. japonicum*. **2. Why Other Options are Incorrect:** * **Amoebiasis & D. Giardiasis:** These are protozoal infections primarily treated with Nitroimidazoles (e.g., Metronidazole, Tinidazole) or luminal amebicides (e.g., Diloxanide furoate, Paromomycin). * **Leishmaniasis:** This is a protozoal disease treated with Sodium Stibogluconate (Pentavalent antimonials), Amphotericin B, or Miltefosine. **Clinical Pearls for NEET-PG:** * **Drug of Choice:** Although Metrifonate is effective, **Praziquantel** is currently the drug of choice for all species of Schistosomiasis due to its broader spectrum and superior safety profile. * **Side Effects:** Being an organophosphate, Metrifonate can cause cholinergic side effects (nausea, abdominal colic, bronchospasm) and a transient reduction in plasma cholinesterase levels. * **Contraindication:** It should be avoided in patients recently exposed to insecticides or those taking depolarizing neuromuscular blockers like Succinylcholine.
Question 132: Which of the following is used for single dose radical cure of Plasmodium vivax malaria?
- A. Tafenoquine (Correct Answer)
- B. Elagolix sodium
- C. Ibalizumab
- D. Patisiran
Explanation: **Explanation:** **Tafenoquine** is the correct answer because it is a long-acting 8-aminoquinoline derivative recently approved for the **radical cure** (prevention of relapse) of *Plasmodium vivax* malaria. Unlike Primaquine, which requires a 14-day course, Tafenoquine has a prolonged half-life (approx. 14 days), allowing for a **single-dose** regimen to eliminate dormant liver stages (**hypnozoites**). **Analysis of Incorrect Options:** * **Elagolix sodium:** A gonadotropin-releasing hormone (GnRH) receptor antagonist used for the management of pain associated with endometriosis. * **Ibalizumab:** A humanized monoclonal antibody that acts as a post-attachment inhibitor for the treatment of multi-drug resistant HIV-1. * **Patisiran:** A small interfering RNA (siRNA) used for the treatment of polyneuropathy caused by hereditary transthyretin-mediated (hATTR) amyloidosis. **Clinical Pearls for NEET-PG:** 1. **G6PD Screening:** Like Primaquine, Tafenoquine causes oxidative hemolysis. Quantitative **G6PD activity testing is mandatory** before administration; it is contraindicated in patients with <70% enzyme activity. 2. **Mechanism:** It targets both pre-erythrocytic (liver) and erythrocytic (blood) stages, as well as gametocytes. 3. **Contraindications:** Avoid in pregnancy, lactation (unless the infant is tested for G6PD), and patients with a history of psychotic disorders. 4. **Comparison:** While Primaquine is the traditional choice for radical cure, **Tafenoquine’s single-dose profile** significantly improves patient compliance.
Question 133: Tetracycline is contraindicated in:
- A. Children
- B. Pregnancy
- C. Renal failure
- D. All the above (Correct Answer)
Explanation: **Explanation:** Tetracyclines are broad-spectrum bacteriostatic antibiotics that inhibit protein synthesis by binding to the 30S ribosomal subunit. Their contraindications are high-yield topics for NEET-PG due to their specific adverse effect profile: 1. **Children (<8 years):** Tetracyclines have a high affinity for calcium. They deposit in growing teeth and bones as **calcium-orthophosphate complexes**, leading to permanent **yellowish-brown discoloration of teeth** and enamel hypoplasia. They can also cause temporary suppression of bone growth. 2. **Pregnancy:** These drugs are classified as **FDA Category D**. They cross the placenta and can cause fetal teeth/bone abnormalities. Additionally, pregnant women are at an increased risk of **acute fatty liver necrosis** (hepatotoxicity) when administered high doses of tetracyclines. 3. **Renal Failure:** Most tetracyclines (except Doxycycline and Minocycline) are excreted via the kidneys. In renal impairment, they accumulate and exert an **anti-anabolic effect**, increasing urea nitrogen (BUN) and aggravating azotemia. **High-Yield Clinical Pearls for NEET-PG:** * **The Exception:** **Doxycycline** is the safest tetracycline in renal failure because it is primarily excreted via bile (fecal route). * **Fanconi Syndrome:** Use of **expired tetracyclines** leads to proximal renal tubular acidosis (Fanconi Syndrome) due to degradation products like epianhydrotetracycline. * **Phototoxicity:** Tetracyclines (especially Demeclocycline) can cause exaggerated sunburn reactions. * **SIADH:** Demeclocycline is used therapeutically to treat SIADH because it inhibits ADH action in the collecting ducts.
Question 134: Which of the following cephalosporins can cause thrombocytopenia?
- A. Cefepime
- B. Cefactor
- C. Ceftazidime (Correct Answer)
- D. Ceftobiprole
Explanation: **Explanation:** **Ceftazidime** is a third-generation cephalosporin primarily known for its potent activity against *Pseudomonas aeruginosa*. Among the cephalosporins, it is specifically associated with hematological adverse effects, including **thrombocytopenia**, leukopenia, and eosinophilia. While the exact mechanism is often immune-mediated (drug-induced immune thrombocytopenia), it is a recognized clinical side effect that requires monitoring during prolonged therapy. **Analysis of Options:** * **Cefepime (Option A):** A fourth-generation cephalosporin. Its most characteristic high-yield side effect is **neurotoxicity** (encephalopathy, seizures), especially in patients with renal impairment. * **Cefaclor (Option B):** A second-generation oral cephalosporin. It is classically associated with **serum sickness-like reactions** in children (arthralgia, fever, and urticaria). * **Ceftobiprole (Option D):** A fifth-generation cephalosporin with MRSA activity. While it can cause standard GI upset or taste disturbances, it is not the classic answer for drug-induced thrombocytopenia in this context. **High-Yield Clinical Pearls for NEET-PG:** * **Disulfiram-like reaction & Bleeding (Hypoprothrombinemia):** Associated with cephalosporins containing the **Methylthiotetrazole (MTT) side chain** (e.g., Cefoperazone, Cefotetan). These inhibit Vitamin K epoxide reductase. * **Biliary Sludging:** Classically associated with **Ceftriaxone** due to its high biliary excretion. * **Anti-Pseudomonal Cephalosporins:** Ceftazidime (3rd gen) and Cefepime (4th gen). * **MRSA Coverage:** Ceftaroline and Ceftobiprole (5th gen).
Question 135: The primary reason for the use of drug combination in the treatment of tuberculosis is to:
- A. Ensure patient compliance with the drug regimen
- B. Enhance activity against metabolically inactive mycobacteria
- C. Delay or prevent the emergence of resistance (Correct Answer)
- D. Provide prophylaxis against other bacterial infections
Explanation: **Explanation:** **1. Why Option C is correct:** The hallmark of *Mycobacterium tuberculosis* is its ability to undergo spontaneous chromosomal mutations that lead to drug resistance. In a typical cavitary lung lesion, there are approximately $10^7$ to $10^9$ bacilli. The probability of a mutation conferring resistance to Isoniazid is roughly 1 in $10^6$, and for Rifampicin, it is 1 in $10^8$. If a single drug is used, the few resistant mutants will survive, multiply, and eventually dominate the population (**"selective pressure"**). However, the probability of a bacterium developing resistance to *both* drugs simultaneously is the product of individual probabilities (e.g., $10^{-6} \times 10^{-8} = 10^{-14}$). Since the total bacterial load in a patient is much lower than $10^{14}$, using a combination effectively **prevents the emergence of multi-drug resistant (MDR) strains.** **2. Why other options are incorrect:** * **Option A:** Drug combinations (especially if not in Fixed-Dose Combinations) often increase the pill burden, which can actually **decrease** patient compliance. * **Option B:** While specific drugs like Pyrazinamide target semi-dormant bacilli in acidic environments, the primary *systemic* reason for using a "combination" (multiple drugs) is resistance prevention, not just metabolic coverage. * **Option D:** Antitubercular therapy (ATT) is highly specific. It is not designed to provide prophylaxis against unrelated bacterial infections. **3. High-Yield Clinical Pearls for NEET-PG:** * **Monotherapy** is only acceptable in **Latent TB** (e.g., Isoniazid for 6–9 months). * **Multi-Drug Resistant TB (MDR-TB):** Resistance to at least Isoniazid and Rifampicin. * **Extensively Drug-Resistant TB (XDR-TB):** MDR-TB plus resistance to any fluoroquinolone and at least one second-line injectable (or newer drugs like Bedaquiline/Linezolid per updated WHO definitions). * **Sterilizing activity:** Rifampicin and Pyrazinamide are the most potent sterilizing agents, crucial for shortening the duration of therapy.
Question 136: Which of the following antimicrobials can be used for the treatment of diabetic gastroparesis?
- A. Erythromycin (Correct Answer)
- B. Amikacin
- C. Doxycycline
- D. Metronidazole
Explanation: ### Explanation **Correct Option: A. Erythromycin** Erythromycin, a macrolide antibiotic, acts as a **Motilin receptor agonist**. Motilin is a peptide hormone that stimulates the Migrating Motor Complex (MMC), promoting gastrointestinal motility. By mimicking motilin, Erythromycin induces strong antral contractions, which facilitates gastric emptying. This "prokinetic" effect is clinically utilized in the management of **diabetic gastroparesis** (delayed gastric emptying) and post-operative ileus [1]. **Why other options are incorrect:** * **B. Amikacin:** This is an aminoglycoside used for aerobic Gram-negative infections. It has no effect on GI motility and is associated with nephrotoxicity and ototoxicity. * **C. Doxycycline:** A tetracycline used for atypical pneumonias, rickettsial infections, and acne. It does not possess prokinetic properties. * **D. Metronidazole:** An antiprotozoal and anaerobic antibacterial agent. While used for *C. difficile* or *H. pylori*, it does not stimulate gastric motility. **High-Yield Clinical Pearls for NEET-PG:** * **Tachyphylaxis:** The prokinetic effect of Erythromycin is short-lived due to the rapid downregulation of motilin receptors; hence, it is usually reserved for acute flares or short-term use. * **Dosage:** The dose required for prokinetic action (approx. 125–250 mg) is lower than the dose required for antimicrobial action. * **Other Prokinetics:** Metoclopramide (D2 antagonist) is often the first-line agent for gastroparesis, but Erythromycin is the most potent stimulant of gastric emptying. * **Side Effects:** Erythromycin is a known inhibitor of **CYP3A4** and can cause QT interval prolongation.
Question 137: Which of the following antimicrobial agents is exclusively used topically?
- A. Streptomycin
- B. Neomycin (Correct Answer)
- C. Gentamycin
- D. Netilmycin
Explanation: **Explanation:** The correct answer is **Neomycin**. **1. Why Neomycin is the correct answer:** Neomycin is an aminoglycoside that is **highly nephrotoxic and ototoxic** when administered systemically. Due to its severe systemic toxicity, its use is strictly restricted to **topical applications** (skin ointments, eye/ear drops) and **oral administration for local action** within the gut. Since it is not absorbed from the gastrointestinal tract, oral neomycin is used for "gut sterilization" before colorectal surgery and to reduce ammonia-producing bacteria in Hepatic Encephalopathy. **2. Why the other options are incorrect:** * **Streptomycin (A):** Primarily used as a first-line drug for Tuberculosis and in the treatment of Plague and Tularemia. It is administered via intramuscular injection. * **Gentamycin (C):** The most commonly used systemic aminoglycoside. It is administered IV/IM for serious Gram-negative infections, sepsis, and complicated UTIs. * **Netilmycin (D):** A semi-synthetic aminoglycoside used systemically. It is often preferred because it is less ototoxic than Gentamycin and is effective against some bacteria resistant to other aminoglycosides. **3. NEET-PG High-Yield Pearls:** * **Framycetin (Soframycin):** Like Neomycin, it is another aminoglycoside used *exclusively* topically. * **Mechanism of Action:** Aminoglycosides inhibit protein synthesis by binding to the **30S ribosomal subunit**, causing mRNA misreading. * **Adverse Effects:** All systemic aminoglycosides share a triad of toxicities: **Nephrotoxicity** (Acute Tubular Necrosis), **Ototoxicity** (Vestibular/Cochlear), and **Neuromuscular blockade**. * **Resistance:** Neomycin is the most common aminoglycoside to cause skin sensitization (allergic contact dermatitis).
Question 138: Linezolid belongs to which class of drugs?
- A. Macrolide
- B. Aminoglycoside
- C. Oxazolidinones (Correct Answer)
- D. Streptogamins
Explanation: **Explanation:** **Linezolid** is the first clinically available member of the **Oxazolidinones** class of synthetic antimicrobial agents. It is primarily used to treat infections caused by multi-drug resistant Gram-positive bacteria. **Mechanism of Action:** Linezolid inhibits protein synthesis by binding to the **23S ribosomal RNA of the 50S subunit**. Its unique mechanism involves preventing the formation of the **70S initiation complex**, which distinguishes it from other protein synthesis inhibitors that typically act on peptide chain elongation. **Analysis of Options:** * **A. Macrolides:** (e.g., Erythromycin, Azithromycin) These also bind to the 50S subunit but act by inhibiting translocation. They have a different chemical structure (macrocyclic lactone ring). * **B. Aminoglycosides:** (e.g., Gentamicin, Amikacin) These bind to the **30S subunit**, causing mRNA misreading. They are bactericidal and primarily target Gram-negative aerobes. * **D. Streptogamins:** (e.g., Quinupristin/Dalfopristin) While they also target the 50S subunit and are used for resistant Gram-positive infections, they belong to a distinct chemical class derived from *Streptomyces*. **High-Yield Clinical Pearls for NEET-PG:** 1. **Spectrum:** Excellent activity against **MRSA** (Methicillin-resistant *S. aureus*) and **VRE** (Vancomycin-resistant *Enterococci*). 2. **Bioavailability:** It has **100% oral bioavailability**, allowing for an easy switch from IV to oral therapy. 3. **Adverse Effects:** Long-term use (>2 weeks) can lead to **bone marrow suppression** (thrombocytopenia is most common) and **peripheral/optic neuropathy**. 4. **Drug Interaction:** Linezolid is a weak non-selective **MAO inhibitor**. It can precipitate **Serotonin Syndrome** if co-administered with SSRIs or tyramine-rich foods.
Question 139: Mebendazole is more effective than albendazole in the treatment of which of the following helminthic infections?
- A. Enterobius vermicularis
- B. Trichuris trichiura (Correct Answer)
- C. Trichinella spiralis
- D. Ascaris lumbricoides
Explanation: **Explanation:** The correct answer is **Trichuris trichiura (Whipworm)**. **Why Trichuris trichiura is correct:** While both Mebendazole and Albendazole are broad-spectrum benzimidazoles that inhibit microtubule synthesis (by binding to β-tubulin), **Mebendazole** is clinically considered superior for **Trichuriasis**. This is primarily due to its slower transit time through the gastrointestinal tract and its local action within the gut lumen where the adult whipworms reside. Clinical studies consistently show higher cure rates and egg reduction rates for Mebendazole compared to a single dose of Albendazole in treating *Trichuris trichiura*. **Analysis of Incorrect Options:** * **Enterobius vermicularis (Pinworm):** Both drugs are equally highly effective (nearly 100% cure rate). Albendazole is often preferred due to its simpler single-dose regimen. * **Trichinella spiralis:** Albendazole is the drug of choice here. Unlike Mebendazole, Albendazole is well-absorbed systemically, allowing it to reach the larvae encysted in muscle tissue. * **Ascaris lumbricoides (Roundworm):** Both drugs are highly effective. Albendazole is generally preferred in mass drug administration programs due to its superior efficacy against hookworms, which often co-infect patients with Ascariasis. **NEET-PG High-Yield Pearls:** * **Mechanism of Action:** Inhibition of microtubule polymerization by binding to β-tubulin, leading to glucose depletion and death of the parasite. * **Absorption:** Albendazole is a prodrug with better systemic absorption (increased when taken with a **fatty meal**), making it the drug of choice for **Hydatid disease** (*Echinococcus*) and **Neurocysticercosis** (*Taenia solium*). * **Teratogenicity:** Both drugs are generally avoided in the first trimester of pregnancy. * **Drug of Choice for Hookworm:** Albendazole is significantly more effective than Mebendazole for *Necator americanus*.
Question 140: A patient with multidrug-resistant tuberculosis is undergoing treatment with anti-tubercular drugs. After a few months, the patient develops red-green color blindness. Which drug is most likely causing this adverse effect?
- A. Rifampicin
- B. Ethambutol (Correct Answer)
- C. Isoniazid
- D. Streptomycin
Explanation: **Explanation:** The correct answer is **Ethambutol**. **1. Why Ethambutol is correct:** Ethambutol is a bacteriostatic anti-tubercular drug that inhibits the enzyme **arabinosyltransferase**, thereby interfering with cell wall synthesis. Its most characteristic and dose-dependent adverse effect is **Retrobulbar Neuritis**. This manifests clinically as a decrease in visual acuity, central scotoma, and specifically, **red-green color blindness** (loss of color perception). This toxicity is usually reversible upon discontinuation of the drug but requires baseline and monthly visual acuity and color vision testing (using Ishihara charts). **2. Why other options are incorrect:** * **Rifampicin:** Known for causing **orange-red discoloration** of body fluids (urine, sweat, tears). It is a potent enzyme inducer and can cause hepatotoxicity and "flu-like syndrome." * **Isoniazid (INH):** The hallmark side effect is **Peripheral Neuropathy** (due to Vitamin B6/Pyridoxine deficiency) and hepatotoxicity. It does not typically affect color vision. * **Streptomycin:** An aminoglycoside that causes **Ototoxicity** (specifically vestibulotoxicity, leading to vertigo and balance issues) and Nephrotoxicity. It does not cause optic neuritis. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Ethambutol:** "**E** for **E**ye" (Optic neuritis). * **Safe in Pregnancy:** Ethambutol is generally considered the safest first-line ATT drug during pregnancy. * **Renal Clearance:** It is primarily excreted by the kidneys; therefore, the dose must be adjusted in patients with renal failure to prevent ocular toxicity. * **Contraindication:** It should be avoided in young children who are unable to report changes in visual acuity accurately.
Practice by Chapter
Beta-Lactam Antibiotics
Practice Questions
Aminoglycosides
Practice Questions
Macrolides and Ketolides
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Tetracyclines
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Quinolones
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Sulfonamides and Trimethoprim
Practice Questions
Antimycobacterial Drugs
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Antifungal Agents
Practice Questions
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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