Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 171: Which of the following anti-TB drugs requires an acidic medium for its action?

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

Explanation: ### Explanation **Correct Option: C. Pyrazinamide** Pyrazinamide (PZA) is a unique prodrug that requires conversion into its active form, **pyrazinoic acid**, by the bacterial enzyme *pyrazinamidase* [1]. This conversion and the subsequent accumulation of the drug within the mycobacteria occur most efficiently in an **acidic environment (pH 5.0–5.5)** [1]. In the body, this acidic medium is found within the **phagolysosomes of macrophages**, where *M. tuberculosis* organisms are often sequestered [1]. Because it targets intracellular bacilli in acidic environments, Pyrazinamide is highly effective against "slowly multiplying" or "persister" organisms, giving it a potent **sterilizing effect** that allows for the shortening of TB treatment from 9 months to 6 months. **Incorrect Options:** * **A. Isoniazid:** Acts by inhibiting mycolic acid synthesis. It is most effective against rapidly dividing extracellular bacilli and works best at a neutral pH. * **B. Rifampicin:** Inhibits DNA-dependent RNA polymerase. It is bactericidal against both intracellular and extracellular organisms and is relatively independent of pH for its primary mechanism. * **C. Ethambutol:** A bacteriostatic agent that inhibits arabinosyltransferase. It works best at a neutral pH and is primarily used to prevent the emergence of resistance. **High-Yield NEET-PG Pearls:** * **Site of Action:** Intracellular (Macrophage phagolysosomes). * **Most Common Side Effect:** Hyperuricemia (due to inhibition of uric acid excretion), which may precipitate acute gouty arthritis. * **Hepatotoxicity:** Pyrazinamide is considered the most hepatotoxic drug among the first-line anti-TB agents. * **Resistance:** Mutation in the *pncA* gene (which encodes pyrazinamidase) is the primary mechanism of resistance.

Question 172: Which cephalosporin is used to treat Pseudomonas aeruginosa infections?

A. Cefadroxil
B. Cefuroxime
C. Ceftazidime (Correct Answer)
D. Cefotaxime

Explanation: **Explanation:** The correct answer is **Ceftazidime**. **1. Why Ceftazidime is correct:** Ceftazidime is a **3rd-generation cephalosporin** specifically distinguished by its potent activity against *Pseudomonas aeruginosa*. While most 3rd-generation cephalosporins focus on Gram-negative coverage, ceftazidime (along with the 4th-generation Cefepime) possesses a specific side chain that enhances its penetration through the outer membrane of *Pseudomonas* and increases its stability against its beta-lactamases. **2. Why the other options are incorrect:** * **Cefadroxil (Option A):** A **1st-generation** cephalosporin. It is primarily used for Gram-positive cocci (like *Staph* and *Strept*) and basic urinary tract infections (*E. coli*). It has no activity against *Pseudomonas*. * **Cefuroxime (Option B):** A **2nd-generation** cephalosporin. It offers improved Gram-negative coverage (e.g., *H. influenzae*) compared to the 1st generation but lacks the spectrum required to treat *Pseudomonas*. * **Cefotaxime (Option D):** A **3rd-generation** cephalosporin. While it has excellent coverage against most Gram-negative bacteria and is a drug of choice for bacterial meningitis, it is notoriously **inactive** against *Pseudomonas aeruginosa*. **3. Clinical Pearls for NEET-PG:** * **Anti-Pseudomonal Cephalosporins:** Remember the "3-4-5 rule": **Ceftazidime** (3rd gen), **Cefepime** (4th gen), and **Ceftobiprole/Ceftolozane** (5th gen). * **Ceftazidime Side Effect:** It is associated with a higher risk of inducing resistance (via AmpC beta-lactamases) and can rarely cause neurotoxicity/seizures in renal failure. * **Mnemonic:** "Taz" (Ceftazidime) and "Pime" (Cefepime) fight the "Pseudomonas" slime.

Question 173: Overexpression of the InhA gene can lead to cross-resistance of isoniazid with which of the following drugs?

A. Rifampicin
B. Ethambutol
C. Ethionamide (Correct Answer)
D. Para-aminosalicylic acid (PAS)

Explanation: **Explanation:** The correct answer is **Ethionamide**. **Mechanism of Cross-Resistance:** Both Isoniazid (INH) and Ethionamide are prodrugs that inhibit mycolic acid synthesis in the mycobacterial cell wall. Their primary molecular target is the **InhA enzyme** (enoyl-ACP reductase). * **Isoniazid** is activated by the catalase-peroxidase enzyme (**KatG**). * **Ethionamide** is activated by the monooxygenase enzyme (**EthA**). While they require different enzymes for activation, they share the same final target (InhA). Resistance to Isoniazid can occur via two main mutations: 1. **KatG mutation:** Most common; causes high-level resistance to INH but *no* cross-resistance with Ethionamide. 2. **InhA overexpression/mutation:** Causes low-level resistance to INH and **cross-resistance with Ethionamide**, because the overexpressed target enzyme overwhelms both drugs. **Analysis of Incorrect Options:** * **A. Rifampicin:** Acts by inhibiting DNA-dependent RNA polymerase (*rpoB* gene mutation). It has no structural or target similarity with INH. * **B. Ethambutol:** Inhibits arabinosyl transferase (*embB* gene), interfering with arabinogalactan synthesis. * **D. PAS:** Inhibits dihydropteroate synthase, interfering with folate synthesis (similar to sulfonamides). **High-Yield Clinical Pearls for NEET-PG:** * **KatG mutation** is the most frequent cause of Isoniazid resistance. * **InhA mutation** is the specific mechanism that links INH resistance to Ethionamide resistance. * **Mnemonic:** "InhA" = **I**soniazid a**n**d **H**-ethionamide (Ethionamide). * Isoniazid is the drug of choice for Latent TB and Chemoprophylaxis. Its most common side effect is peripheral neuropathy (prevented by Vitamin B6/Pyridoxine).

Question 174: Which of the following is a bactericidal drug for Mycobacterium leprae?

A. Ofloxacin (Correct Answer)
B. Ciprofloxacin
C. Amoxicillin
D. Erythromycin

Explanation: ### Explanation **Correct Option: A. Ofloxacin** Ofloxacin is a fluoroquinolone that acts by inhibiting the enzyme **DNA gyrase (Topoisomerase II)**, preventing bacterial DNA replication. In the context of *Mycobacterium leprae*, Ofloxacin exhibits potent **bactericidal** activity. It is highly effective against the bacilli and is used as a component of alternative MDT (Multidrug Therapy) regimens, particularly in cases of Rifampicin resistance or intolerance. **Analysis of Incorrect Options:** * **B. Ciprofloxacin:** While also a fluoroquinolone, Ciprofloxacin has poor penetration into tissues for *M. leprae* and shows inconsistent activity compared to Ofloxacin. It is generally not used in standard leprosy protocols. * **C. Amoxicillin:** This is a beta-lactam antibiotic. *M. leprae* produces beta-lactamases, making most penicillins ineffective. Furthermore, cell wall synthesis inhibitors like Amoxicillin do not show significant bactericidal activity against the slow-growing leprosy bacilli. * **D. Erythromycin:** This macrolide is bacteriostatic for many organisms and has negligible activity against *M. leprae*. However, another macrolide, **Clarithromycin**, is highly bactericidal against *M. leprae* and is used in alternative regimens. **High-Yield Clinical Pearls for NEET-PG:** * **First-line Bactericidal Drugs for Leprosy:** Rifampicin (most potent), Ofloxacin, and Clarithromycin. * **Bacteriostatic Drugs for Leprosy:** Dapsone and Ethionamide. * **Clofazimine:** Primarily bacteriostatic but possesses weak bactericidal activity and important anti-inflammatory properties (useful in Type 2 Lepra reactions). * **ROM Regimen:** A single dose of **R**ifampicin (600 mg), **O**floxacin (400 mg), and **M**inocycline (100 mg) was previously used for Single Lesion Paucibacillary (SLPB) leprosy.

Question 175: What is the treatment of choice for Legionnaires' disease?

A. Tetracycline
B. Erythromycin (Correct Answer)
C. Penicillin
D. Streptomycin

Explanation: **Explanation:** **Legionnaires' disease** is a severe form of pneumonia caused by *Legionella pneumophila*, an aerobic, Gram-negative intracellular bacterium. **Why Erythromycin is the Correct Answer:** The treatment of choice for Legionnaires' disease historically and classically (as per standard textbooks like K.D. Tripathi) is **Erythromycin**, a Macrolide antibiotic. Macrolides are effective because they are lipophilic and achieve high intracellular concentrations, which is essential for eradicating *Legionella*—an organism that resides and multiplies within alveolar macrophages. While newer macrolides like Azithromycin or Fluoroquinolones (Levofloxacin) are now preferred in modern clinical practice due to better tolerability, Erythromycin remains the "textbook" answer for examinations. **Why Incorrect Options are Wrong:** * **Tetracycline (A):** While Doxycycline has some activity against *Legionella*, it is considered a second-line agent and is less effective than Macrolides. * **Penicillin (C):** *Legionella* produces beta-lactamases, making it inherently resistant to Penicillins. Furthermore, Penicillins do not achieve sufficient intracellular penetration. * **Streptomycin (D):** This Aminoglycoside is primarily used for Tuberculosis and Plague. It lacks the necessary intracellular penetration to target *Legionella*. **High-Yield NEET-PG Pearls:** * **Source:** Outbreaks are often linked to contaminated water systems, AC cooling towers, and nebulizers. * **Clinical Clue:** Look for pneumonia associated with **hyponatremia**, diarrhea, and high fever (relative bradycardia). * **Diagnosis:** The most rapid test is the **Urinary Antigen Test** (detects Serogroup 1). * **Drug of Choice (Modern):** If Erythromycin is not in the options, look for **Azithromycin** or **Levofloxacin**.

Question 176: All of the following drugs are protease inhibitors, EXCEPT:

A. Nelfinavir
B. Saquinavir
C. Abacavir (Correct Answer)
D. Ritonavir

Explanation: **Explanation:** The correct answer is **Abacavir (Option C)**. To answer this question, one must distinguish between the different classes of Antiretroviral Therapy (ART). 1. **Why Abacavir is the correct answer:** Abacavir belongs to the **Nucleoside Reverse Transcriptase Inhibitors (NRTIs)** class, not Protease Inhibitors. It works by competitively inhibiting the reverse transcriptase enzyme and acting as a chain terminator during viral DNA synthesis. A high-yield clinical fact regarding Abacavir is its association with a severe **hypersensitivity reaction** linked to the **HLA-B*5701** allele; screening is mandatory before initiation. 2. **Why the other options are incorrect:** * **Nelfinavir, Saquinavir, and Ritonavir** are all **Protease Inhibitors (PIs)**. * **Mechanism:** PIs bind to the active site of the HIV protease enzyme, preventing the cleavage of gag-pol polyproteins into functional mature proteins. This results in the production of immature, non-infectious virions. * **Mnemonic:** Most Protease Inhibitors end with the suffix **"-navir"** (e.g., Atazanavir, Darunavir, Lopinavir). **High-Yield NEET-PG Clinical Pearls:** * **Ritonavir** is rarely used for its own antiviral activity; it is primarily used as a **"pharmacokinetic booster"** because it is a potent inhibitor of the CYP3A4 enzyme, thereby increasing the plasma levels of other PIs. * **Common Side Effects of PIs:** Lipodystrophy (buffalo hump), insulin resistance (hyperglycemia), and hyperlipidemia. * **Indinavir** is specifically associated with nephrolithiasis (kidney stones).

Question 177: Which drugs are known to cause ototoxicity and circumoral paresthesia?

A. Antileprotic drugs
B. Antitubercular drugs
C. Streptomycin (Correct Answer)
D. Chloramphenicol

Explanation: **Explanation:** **Streptomycin** is an aminoglycoside antibiotic primarily used in the treatment of tuberculosis. Its side effect profile is a high-yield topic for NEET-PG. 1. **Why Streptomycin is correct:** * **Ototoxicity:** Aminoglycosides are notoriously vestibulotoxic and cochleotoxic. Streptomycin specifically tends to affect the vestibular apparatus more than the cochlea, leading to vertigo and loss of balance. * **Circumoral Paresthesia:** This is a classic, early neurological side effect of streptomycin. It occurs due to the drug's effect on peripheral nerves or transient electrolyte disturbances. It typically manifests as numbness or tingling around the mouth shortly after injection. 2. **Why other options are incorrect:** * **Antileprotic drugs:** Drugs like Dapsone are associated with hemolysis (in G6PD deficiency) and "Dapsone syndrome," while Clofazimine causes skin discoloration. They do not typically cause ototoxicity. * **Antitubercular drugs:** While this is a broad category that *includes* streptomycin, the question asks for specific drug effects. Other first-line drugs like Ethambutol (optic neuritis) or Isoniazid (peripheral neuropathy) do not cause ototoxicity. * **Chloramphenicol:** This drug is primarily associated with Bone Marrow Suppression (dose-dependent) and the potentially fatal **Gray Baby Syndrome** in neonates. **High-Yield Clinical Pearls for NEET-PG:** * **Aminoglycoside Rule:** All aminoglycosides are nephrotoxic and ototoxic. * **Specific Toxicity:** Streptomycin and Gentamicin are more **vestibulotoxic**; Amikacin and Neomycin are more **cochleotoxic**. * **Contraindication:** Streptomycin is contraindicated in pregnancy because it can cross the placenta and cause **permanent bilateral deafness** in the fetus. * **Mechanism:** They inhibit protein synthesis by binding to the **30S ribosomal subunit**.

Question 178: Isoniazid and pyridoxine are given together in antituberculous chemotherapy:

A. To prevent peripheral neuritis (Correct Answer)
B. To prevent emergence of isoniazid resistance
C. As a nutrient supplement
D. As a synergistic combination

Explanation: **Explanation:** **1. Why Option A is Correct:** Isoniazid (INH) is a structural analogue of **Pyridoxine (Vitamin B6)**. It causes a deficiency of B6 through two mechanisms: it inhibits the enzyme *pyridoxine phosphokinase* (which converts B6 to its active form, pyridoxal phosphate) and increases the renal excretion of B6 by forming isoniazid-pyridoxal hydrazones. Pyridoxal phosphate is a vital co-factor for the synthesis of inhibitory neurotransmitters like GABA. Its deficiency leads to **peripheral neuropathy** (paresthesia, numbness, and "burning feet" syndrome). Administering prophylactic Pyridoxine (10–50 mg/day) prevents this neurotoxicity. **2. Why Other Options are Incorrect:** * **Option B:** Resistance to INH is prevented by using it in combination with other first-line ATT drugs (Rifampicin, Pyrazinamide, Ethambutol), not by adding vitamins. * **Option C:** While Pyridoxine is a nutrient, it is specifically added here as a pharmacological intervention to counteract drug-induced toxicity, not for general nutritional supplementation. * **Option D:** Synergism refers to enhanced antimicrobial efficacy. Pyridoxine does not increase the bactericidal activity of INH; it only reduces side effects. **High-Yield Clinical Pearls for NEET-PG:** * **Risk Factors:** Peripheral neuritis is more common in **slow acetylators**, diabetics, alcoholics, pregnant women, and malnourished patients. * **Sideroblastic Anemia:** INH can also cause microcytic anemia because B6 is a cofactor for ALA synthase (the rate-limiting step in heme synthesis). * **Other Side Effects:** Hepatotoxicity (most common), Drug-induced Lupus (anti-Histone antibodies), and hemolysis in G6PD deficiency.

Question 179: Which of the following drugs is active selectively against anaerobic and microaerophilic organisms?

A. Clindamycin
B. Tetracycline
C. Metronidazole (Correct Answer)
D. Aminoglycosides

Explanation: **Explanation:** **Metronidazole** is the correct answer because its mechanism of action is uniquely dependent on the anaerobic environment. It is a prodrug that requires **reductive activation** of its nitro group by the enzyme **pyruvate:ferredoxin oxidoreductase (PFOR)**, which is found only in anaerobic and microaerophilic bacteria and protozoa. Once reduced, it forms highly reactive cytotoxic intermediates that cause DNA strand breakage and cell death. Aerobic organisms are resistant because oxygen competes for electrons and prevents the reduction of the drug. **Analysis of Incorrect Options:** * **Clindamycin (A):** While it has excellent activity against anaerobes (especially above the diaphragm), it is also active against many aerobic Gram-positive cocci (e.g., MRSA, Streptococci). It is not *selectively* anaerobic. * **Tetracycline (B):** These are broad-spectrum antibiotics active against a wide range of aerobic and anaerobic Gram-positive and Gram-negative bacteria, as well as atypical organisms (Rickettsia, Chlamydia). * **Aminoglycosides (D):** These are the polar opposite; they are **ineffective** against anaerobes. Their uptake into the bacterial cell is an oxygen-dependent process, making them active only against aerobic Gram-negative bacilli. **High-Yield NEET-PG Pearls:** * **Mnemonic for Metronidazole:** "GET GAP" (Giardia, Entamoeba, Trichomonas, Gardnerella, Anaerobes [B. fragilis], and PUD [H. pylori]). * **Drug of Choice:** For *Clostridioides difficile* (mild-to-moderate) and bacterial vaginosis. * **Adverse Effects:** Disulfiram-like reaction with alcohol and a metallic taste in the mouth. * **Clinical Rule:** Clindamycin for anaerobes "above the diaphragm"; Metronidazole for anaerobes "below the diaphragm."

Question 180: Lamivudine acts at which of the following enzymes?

A. DNA polymerase
B. Reverse transcriptase
C. Both of the above (Correct Answer)
D. None of the above

Explanation: **Explanation:** Lamivudine (3TC) is a nucleoside analog that acts as a potent inhibitor of viral replication. Its mechanism of action involves being phosphorylated intracellularly into its active triphosphate form, which then competes with natural nucleotides for incorporation into viral DNA. **Why "Both of the above" is correct:** Lamivudine is unique because it is used to treat two distinct viral infections by targeting structurally similar enzymes: 1. **Reverse Transcriptase (RNA-dependent DNA polymerase):** In **HIV**, Lamivudine acts as a Nucleoside Reverse Transcriptase Inhibitor (NRTI). It causes premature chain termination of the proviral DNA string. 2. **DNA Polymerase:** In **Hepatitis B Virus (HBV)**, it inhibits the HBV DNA polymerase. Since HBV is a DNA virus that replicates via an RNA intermediate, its DNA polymerase also possesses reverse transcriptase activity, making it highly sensitive to Lamivudine. **Analysis of incorrect options:** * **Option A & B:** While Lamivudine does inhibit both enzymes, selecting only one would be incomplete. In the context of NEET-PG, when a drug has dual indications (HIV and HBV), its action on both the specific viral DNA polymerase and reverse transcriptase must be acknowledged. **High-Yield Clinical Pearls for NEET-PG:** * **Dual Coverage:** Lamivudine is a first-line component of ART for HIV and is also used for Chronic Hepatitis B. * **Resistance:** The most common mutation associated with Lamivudine resistance is the **M184V** mutation in HIV and the **YMDD** motif mutation in HBV. * **Safety:** It is one of the least toxic NRTIs; however, it can cause a "flare-up" of Hepatitis B if discontinued abruptly in co-infected patients. * **Dosing:** The dose used for HBV (100 mg) is lower than the dose used for HIV (300 mg).

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free