Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 371: The toxicity of ethambutol includes which of the following?

A. Peripheral neuritis
B. Green color blindness (Correct Answer)
C. Red urine
D. Yellow color blindness

Explanation: **Explanation:** Ethambutol is a first-line antitubercular drug (ATT) known for its specific adverse effect profile, primarily targeting the eyes. **Why Option B is Correct:** The most significant toxicity of ethambutol is **retrobulbar neuritis**. This manifests as a decrease in visual acuity and, characteristically, **red-green color blindness** (dyschromatopsia). Patients often lose the ability to distinguish between red and green before visual acuity significantly drops. The mechanism is thought to involve the chelation of copper, which interferes with mitochondrial function in the optic nerve. **Why the Other Options are Incorrect:** * **A. Peripheral neuritis:** This is the classic side effect of **Isoniazid (INH)** due to interference with Vitamin B6 (pyridoxine) metabolism. While ethambutol can rarely cause it, it is not the defining toxicity. * **C. Red urine:** This is a harmless side effect of **Rifampicin**, which causes orange-red discoloration of urine, sweat, and tears. * **D. Yellow color blindness:** Ethambutol specifically affects the red-green axis; yellow-blue disturbances are not characteristic of this drug. **NEET-PG High-Yield Pearls:** 1. **Dose-dependency:** Ethambutol toxicity is dose-related (more common at >25 mg/kg). 2. **Monitoring:** Patients on ethambutol must undergo baseline and monthly **Snellen’s chart** and **Ishihara chart** testing. 3. **Pediatric Contraindication:** It is generally avoided in children below 6 years because they cannot reliably report changes in visual acuity or color perception. 4. **Renal Clearance:** Ethambutol is primarily excreted by the kidneys; doses must be adjusted in renal failure to prevent toxicity. 5. **Bacteriostatic:** Unlike other first-line ATT (HRZ), Ethambutol is primarily bacteriostatic.

Question 372: Mebendazole is used in all of the following conditions except:

A. Cysticercosis
B. Ascariasis
C. Trichuriasis
D. Schistosomiasis (Correct Answer)

Explanation: **Explanation:** The correct answer is **Schistosomiasis** because Mebendazole is an anthelmintic agent belonging to the **Benzimidazole** class, which is primarily effective against **nematodes (roundworms)** and certain larval stages of cestodes. It works by inhibiting microtubule synthesis (binding to β-tubulin), leading to glucose depletion and death of the parasite. **Why Schistosomiasis is the exception:** Schistosomiasis (Bilharzia) is caused by blood flukes, which are **trematodes**. Benzimidazoles like Mebendazole have no clinical activity against trematodes. The drug of choice for Schistosomiasis is **Praziquantel**, which works by increasing the permeability of the parasite cell membrane to calcium, causing paralysis. **Analysis of other options:** * **Ascariasis (B) and Trichuriasis (C):** Mebendazole is a first-line treatment for these common intestinal nematode infections (Roundworm and Whipworm, respectively). * **Cysticercosis (A):** While **Albendazole** is the preferred benzimidazole for Neurocysticercosis due to better CNS penetration, Mebendazole is pharmacologically active against the larval forms of *Taenia solium* and can be used in systemic cysticercosis. **NEET-PG High-Yield Pearls:** * **Mechanism of Action:** Inhibits microtubule polymerization by binding to **β-tubulin**. * **Spectrum:** Mebendazole is "Broad Spectrum" for **Nematodes** (Hookworm, Pinworm, Roundworm, Whipworm). * **Pharmacokinetics:** Mebendazole has poor systemic absorption (useful for intestinal worms), whereas Albendazole has better absorption (enhanced by a fatty meal), making it superior for tissue infections like Hydatid disease. * **Teratogenicity:** Benzimidazoles are generally avoided in the first trimester of pregnancy.

Question 373: The bacterial resistance to tetracycline is due to which of the following mechanisms?

A. Alteration of drug binding sites
B. Alteration of dehydrofolate reductase
C. Inactivation of drug by enzymes (Correct Answer)
D. Decreased uptake of drug by bacteria

Explanation: **Explanation:** The primary mechanism of resistance to **Tetracyclines** is the **efflux of the drug** out of the bacterial cell or **ribosomal protection proteins**. However, according to the specific options provided in this question, the correct mechanism highlighted is the **inactivation of the drug by enzymes**. 1. **Why Option C is correct:** While efflux is most common, certain bacteria (like *Bacteroides fragilis*) produce an enzyme called **tetracycline-destructive enzyme** (an acetyltransferase) that chemically modifies and inactivates the drug, rendering it ineffective. 2. **Why other options are incorrect:** * **Option A:** Alteration of binding sites (the 30S ribosome) is a common mechanism for Aminoglycosides, but not the primary mechanism for Tetracyclines. * **Option B:** Alteration of dihydrofolate reductase (DHFR) is the specific mechanism of resistance for **Trimethoprim** and Methotrexate. * **Option D:** While decreased uptake can occur, it is less clinically significant compared to active efflux pumps (which move the drug out after it has entered). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Tetracyclines are bacteriostatic; they bind to the **30S ribosomal subunit** and inhibit the binding of aminoacyl-tRNA to the A site. * **Most Common Resistance:** The most frequent mechanism is **active efflux** (via *tetA* gene) and **ribosomal protection** (via *tetM* proteins). * **Tigecycline:** This is a Glycylcycline designed to overcome these resistance mechanisms (efflux and ribosomal protection), though it is not effective against *Proteus* or *Pseudomonas*. * **Drug of Choice:** Tetracyclines (specifically Doxycycline) remain the drug of choice for Rickettsial infections, Chlamydia, and Cholera.

Question 374: A person is being treated for Human Immunodeficiency Virus–1 and developed hypertriglyceridemia and hypercholesterolemia. Which drug is most likely implicated for these adverse effects?

A. Ritonavir (Correct Answer)
B. Raltegravir
C. Didanosine
D. Efavirenz

Explanation: **Explanation:** The patient is experiencing **metabolic syndrome** (dyslipidemia), a classic adverse effect associated with **Protease Inhibitors (PIs)**. **1. Why Ritonavir is correct:** Ritonavir belongs to the Protease Inhibitor class. PIs are notorious for causing metabolic complications, including **hyperlipidemia** (elevated triglycerides and cholesterol), **insulin resistance**, and **lipodystrophy** (buffalo hump and peripheral fat wasting). The mechanism involves the inhibition of proteins that regulate lipid metabolism (like LRP and CRABP-1), leading to decreased peripheral storage and increased serum levels of lipids. Ritonavir, often used as a "booster" due to its potent CYP3A4 inhibition, is frequently implicated in these metabolic derangements. **2. Why the other options are incorrect:** * **Raltegravir (Integrase Inhibitor):** Generally considered "metabolically neutral." It is preferred in patients with pre-existing dyslipidemia or cardiovascular risk. * **Didanosine (NRTI):** Primarily associated with **pancreatitis** and peripheral neuropathy. While NRTIs can cause lactic acidosis/hepatic steatosis, they are not the primary cause of isolated hypercholesterolemia. * **Efavirenz (NNRTI):** Most famous for **CNS side effects** (vivid dreams, dizziness, psychosis) and teratogenicity (neural tube defects). While it can cause mild lipid elevations, the effect is significantly less pronounced than with PIs like Ritonavir. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for PI side effects:** "The **PI**s make you **P**lump and **I**nsulin resistant" (Hyperlipidemia, Hyperglycemia, Lipodystrophy). * **Atazanavir:** The PI least likely to cause dyslipidemia ("A-lipidemic"). * **Indinavir:** Associated with nephrolithiasis (crystalluria). * **Zidovudine (AZT):** Causes bone marrow suppression (anemia/neutropenia).

Question 375: Which of the following is a viral HIV integrase inhibitor?

A. Zidovudine
B. Maraviroc
C. Raltegravir (Correct Answer)
D. Enfuvirtide

Explanation: **Explanation:** **Correct Option: C. Raltegravir** Raltegravir is the first-in-class **Integrase Strand Transfer Inhibitor (INSTI)**. Its mechanism of action involves inhibiting the viral enzyme **integrase**, which is responsible for incorporating the reverse-transcribed viral DNA into the host cell genome. By preventing this integration, the virus cannot replicate. Other drugs in this class include Dolutegravir, Elvitegravir, and Bictegravir. **Incorrect Options:** * **A. Zidovudine (AZT):** This is a **Nucleoside Reverse Transcriptase Inhibitor (NRTI)**. It acts as a thymidine analogue that causes chain termination during the synthesis of viral DNA by reverse transcriptase. It is famously known for causing bone marrow suppression (anemia/neutropenia). * **B. Maraviroc:** This is an **Entry Inhibitor** specifically classified as a **CCR5 Antagonist**. It binds to the CCR5 receptor on the host T-cell, preventing the HIV gp120 protein from attaching. It is only effective against "R5-tropic" virus strains. * **C. Enfuvirtide:** This is a **Fusion Inhibitor**. It binds to the **gp41** subunit of the viral envelope glycoprotein, preventing the fusion of the viral envelope with the host cell membrane. It is administered subcutaneously. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for INSTIs:** All integrase inhibitors contain "**tegra**" (from in**tegra**se) in their name (Ral**tegra**vir, Dolu**tegra**vir). * **Drug of Choice:** Dolutegravir-based regimens are currently the preferred first-line ART (Antiretroviral Therapy) globally due to high efficacy and a high genetic barrier to resistance. * **Side Effects:** INSTIs are generally well-tolerated but can cause an increase in creatine kinase (CK) and, rarely, rhabdomyolysis.

Question 376: What is the treatment of choice for pneumonia caused by Mycoplasma?

A. Azithromycin (Correct Answer)
B. Doxycycline
C. Amoxicillin
D. Ciprofloxacin

Explanation: **Explanation:** **1. Why Azithromycin is the Correct Answer:** *Mycoplasma pneumoniae* is a common cause of **Atypical Pneumonia** (Walking Pneumonia). Because *Mycoplasma* lacks a cell wall, it is inherently resistant to beta-lactam antibiotics. **Macrolides**, specifically **Azithromycin**, are the treatment of choice [1]. They work by inhibiting protein synthesis (binding to the 50S ribosomal subunit). Azithromycin is preferred over Erythromycin [3] due to its superior tissue penetration, longer half-life (allowing once-daily dosing) [1], [5], and better gastrointestinal tolerability [4]. **2. Analysis of Incorrect Options:** * **B. Doxycycline:** This is a Tetracycline and is considered a **second-line** alternative. While effective against *Mycoplasma*, it is generally avoided in children and pregnant women due to effects on bone and teeth. * **C. Amoxicillin:** This is a Beta-lactam that inhibits cell wall synthesis. Since *Mycoplasma* **lacks a cell wall**, Amoxicillin is completely ineffective [2]. * **D. Ciprofloxacin:** While some Fluoroquinolones (like Levofloxacin or Moxifloxacin) are effective against atypical pathogens, Ciprofloxacin has poor activity against *Mycoplasma* and is not a primary choice for community-acquired pneumonia. **3. High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** Cold agglutinin titer (IgM antibodies) is a classic bedside test; PCR is the gold standard. * **Radiology:** "Patchy consolidation" or reticulonodular patterns that look much worse than the patient's clinical presentation. * **Complications:** Bullous myringitis (ear pain), Stevens-Johnson Syndrome, and autoimmune hemolytic anemia. * **Drug of Choice (DOC) Summary:** Macrolides (Azithromycin) are DOC for most atypical infections (*Mycoplasma, Chlamydia, Legionella*).

Question 377: Which of the following beta-lactam antibiotics can be safely used in a patient with a history of allergy to penicillins?

A. Aztreonam (Correct Answer)
B. Cefepime
C. Loracarbef
D. Ceftriaxone

Explanation: **Explanation:** The correct answer is **Aztreonam**. **1. Why Aztreonam is correct:** Aztreonam is a **Monobactam**, characterized by a unique monocyclic beta-lactam ring. Unlike other beta-lactams, it lacks the fused adjacent ring structure (like the thiazolidine ring in penicillins or dihydrothiazine ring in cephalosporins). Due to this structural divergence, there is **no cross-reactivity** between aztreonam and other beta-lactams. It can be safely administered to patients with documented Type-1 hypersensitivity (anaphylaxis) to penicillins. *Note:* The only exception is a specific cross-reactivity with **Ceftazidime**, as both share an identical side chain. **2. Why the other options are incorrect:** * **Cefepime (Option B) & Ceftriaxone (Option D):** These are 4th and 3rd generation cephalosporins, respectively. While the risk of cross-reactivity is low (approx. 1–3%), it is not zero. In patients with a history of severe penicillin allergy, cephalosporins are generally avoided unless specifically indicated and tested. * **Loracarbef (Option C):** This is a Carbacephem (structurally similar to cephalosporins). It carries a similar risk of cross-allergenicity with penicillins. **3. High-Yield NEET-PG Pearls:** * **Spectrum:** Aztreonam is active **only against aerobic Gram-negative bacteria** (including *Pseudomonas*). It has no activity against Gram-positives or anaerobes (the "Magic Bullet" for Gram-negatives). * **Mechanism:** It binds specifically to **PBP-3**, leading to the formation of long filamentous bacteria and cell lysis. * **Safety:** It is a "renal-friendly" alternative to aminoglycosides as it does not cause ototoxicity or nephrotoxicity.

Question 378: Tetracycline when given to pregnant women can cause which of the following?

A. Acute hepatic necrosis
B. Discoloration/staining of hard structures of the fetus
C. Increased intracranial pressure in the infant
D. All of the above (Correct Answer)

Explanation: Tetracyclines are broad-spectrum bacteriostatic antibiotics that are strictly contraindicated during pregnancy (FDA Category D) due to their multi-organ toxicity in both the mother and the fetus. **Why "All of the above" is correct:** * **Acute Hepatic Necrosis (Option A):** Pregnant women are uniquely susceptible to tetracycline-induced hepatotoxicity. High doses (especially IV) can lead to **acute fatty liver of pregnancy**, characterized by microvesicular steatosis and hepatic necrosis, which can be fatal. * **Discoloration of Hard Structures (Option B):** Tetracyclines are chelating agents that bind to calcium hydroxyapatite [2]. They cross the placenta and deposit in fetal bones and deciduous teeth, leading to **permanent brownish-yellow discoloration** and enamel hypoplasia [3]. They can also cause temporary suppression of bone growth (fibula). * **Increased Intracranial Pressure (Option C):** In infants and neonates, tetracyclines can cause **Pseudotumor cerebri** (benign intracranial hypertension), clinically manifesting as a **bulging fontanelle**. **High-Yield Clinical Pearls for NEET-PG:** 1. **Fanconi Syndrome:** Expired tetracyclines undergo degradation to form epitetracycline and anhydrotetracycline, which cause proximal renal tubular acidosis (Fanconi Syndrome). 2. **Phototoxicity:** Tetracyclines (especially Demeclocycline and Doxycycline) can cause exaggerated sunburn reactions [1]. 3. **Diabetes Insipidus:** Demeclocycline is used therapeutically in SIADH because it antagonizes ADH action in the renal tubules [2]. 4. **Drug of Choice:** Doxycycline is the preferred tetracycline in renal failure as it is excreted primarily via bile (fecal route).

Question 379: Lamivudine is recommended for treatment of chronic hepatitis B when?

A. HBeAg positive
B. HBeAg negative
C. ALT > 2 x Upper Limit of Normal (ULN) (Correct Answer)
D. Viral DNA > 10^2 copies/mL

Explanation: **Explanation:** The primary goal of treating Chronic Hepatitis B (CHB) is to prevent disease progression to cirrhosis and hepatocellular carcinoma. Treatment is not initiated for every patient with a positive HBsAg; rather, it is reserved for those with evidence of **active liver inflammation** or significant fibrosis. **Why Option C is Correct:** The level of **Alanine Aminotransferase (ALT)** is a surrogate marker for immune-mediated hepatocyte injury. Guidelines (AASLD/EASL) generally recommend initiating antiviral therapy like Lamivudine when **ALT > 2 x the Upper Limit of Normal (ULN)**. This indicates that the body’s immune system is actively attacking infected hepatocytes, and antiviral intervention at this stage is most effective in achieving seroconversion and histological improvement. **Analysis of Incorrect Options:** * **Options A & B:** HBeAg status (positive or negative) determines the *type* of CHB but is not a standalone criterion for starting treatment. Both HBeAg-positive and HBeAg-negative patients require treatment only if they meet specific DNA and ALT thresholds. * **Option D:** While viral load is a criterion, the threshold is much higher. Treatment is typically considered when HBV DNA is **> 2,000 IU/mL** (for HBeAg-negative) or **> 20,000 IU/mL** (for HBeAg-positive). $10^2$ copies/mL is too low a threshold for initiating therapy. **Clinical Pearls for NEET-PG:** * **Mechanism:** Lamivudine is a nucleoside reverse transcriptase inhibitor (NRTI) that inhibits HBV DNA polymerase. * **Resistance:** A major drawback of Lamivudine is the high rate of resistance due to the **YMDD mutation** (Tyrosine-Methionine-Aspartate-Aspartate motif). * **Preferred Agents:** Due to resistance issues, **Entecavir** and **Tenofovir** are now preferred over Lamivudine as first-line oral agents. * **Pregnancy:** Lamivudine is safe and often used in the third trimester to prevent vertical transmission if the maternal viral load is high.

Question 380: A patient presented with diarrhea and their stool examination showed eggs of Hymenolepis nana. What is the drug of choice?

A. Praziquantel (Correct Answer)
B. Mebendazole
C. Albendazole
D. Pyrantel pamoate

Explanation: ### Explanation **1. Why Praziquantel is the Correct Answer:** *Hymenolepis nana* (dwarf tapeworm) is unique among cestodes because it does not require an intermediate host and can cause **autoinfection**. **Praziquantel** is the drug of choice for *H. nana* infections. * **Mechanism of Action:** It increases the permeability of the parasite cell membrane to calcium ions, leading to massive influx. This causes strong muscular contraction and **spastic paralysis** of the worm, followed by vacuolization and disintegration of the tegument. * **Clinical Note:** Unlike other tapeworms where a single dose suffices, *H. nana* may require a slightly higher dose or repeated treatment due to the presence of internal autoinfection cycles (cysticercoids in the intestinal villi). **2. Why the Other Options are Incorrect:** * **Mebendazole & Albendazole (Benzimidazoles):** These are primarily used for **nematodes** (roundworms, hookworms) by inhibiting microtubule synthesis. While Albendazole has some activity against certain cestodes (like Neurocysticercosis or Hydatid disease), it is not the primary choice for *H. nana* [1]. * **Pyrantel Pamoate:** This is a depolarizing neuromuscular blocker used specifically for **nematodes** (e.g., *Ascaris*, Enterobius). It is ineffective against cestodes (tapeworms) [1]. **3. High-Yield Facts for NEET-PG:** * **H. nana** is the most common cause of all cestode infections worldwide and is the **smallest** tapeworm infecting humans. * **Praziquantel** is the DOC for most Trematodes (Schistosomiasis) and Cestodes (Taeniasis, Diphyllobothriasis), with the notable exception of *Fasciola hepatica* (DOC: Triclabendazole). * **Adverse Effect:** Praziquantel can cause a "Mazzotti-like" reaction when treating heavy parasitic loads due to the release of antigens from dying worms.

Practice by Chapter

Beta-Lactam Antibiotics

Practice Questions

Aminoglycosides

Practice Questions

Macrolides and Ketolides

Practice Questions

Tetracyclines

Practice Questions

Quinolones

Practice Questions

Sulfonamides and Trimethoprim

Practice Questions

Antimycobacterial Drugs

Practice Questions

Antifungal Agents

Practice Questions

Antiviral Drugs

Practice Questions

Antiparasitic Agents

Practice Questions

Principles of Antimicrobial Selection

Practice Questions

Antimicrobial Resistance

Practice Questions

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