Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 911: Which drug acts fastest on the Mycobacterium leprae bacilli?

A. Rifampicin (Correct Answer)
B. Dapsone
C. Kanamycin
D. Clofazimine

Explanation: **Explanation:** **Rifampicin** is the correct answer because it is the most potent and fastest-acting bactericidal drug against *Mycobacterium leprae*. It acts by inhibiting the DNA-dependent RNA polymerase enzyme, thereby halting bacterial protein synthesis. A single dose of 600 mg is capable of killing **99.9% of viable bacilli** within 3 to 7 days. Due to this rapid action, patients typically become non-infectious within a week of starting treatment. **Analysis of Incorrect Options:** * **Dapsone (B):** While it is the backbone of leprosy treatment, it is primarily **bacteriostatic**. It works by inhibiting dihydropteroate synthase (folate synthesis). It acts slowly and requires long-term administration to clear the bacterial load. * **Kanamycin (C):** This is an aminoglycoside occasionally used in drug-resistant cases, but it is not a first-line agent and does not match the rapid bactericidal efficacy of Rifampicin in leprosy. * **Clofazimine (D):** This is a **bacteriostatic/weakly bactericidal** dye used in Multi-Drug Therapy (MDT). Its action is very slow, and it is primarily valued for its anti-inflammatory properties in managing Type 2 Lepra reactions (ENL). **High-Yield Clinical Pearls for NEET-PG:** * **MDT Regimen:** For Paucibacillary (PB) leprosy, treatment lasts 6 months; for Multibacillary (MB) leprosy, it lasts 12 months. * **Rifampicin Side Effect:** Can cause orange-discoloration of urine and secretions (harmless) and hepatotoxicity. * **Clofazimine Side Effect:** Causes brownish-black skin discoloration and ichthyosis. * **Dapsone Side Effect:** Hemolysis (especially in G6PD deficiency) and "Dapsone Syndrome" (exfoliative dermatitis, fever, and lymphadenopathy).

Question 912: Which of the following drugs is NOT used in the treatment of extended drug resistance?

A. Rifampicin (Correct Answer)
B. Isoniazid
C. Moxifloxacin
D. Capreomycin

Explanation: The question focuses on the definitions and treatment protocols for drug-resistant tuberculosis (TB), a high-yield topic for NEET-PG. ### **Explanation of the Correct Answer** **A. Rifampicin** is the correct answer because, by definition, **Extensively Drug-Resistant TB (XDR-TB)** is defined as TB that is resistant to at least **Isoniazid and Rifampicin** (MDR-TB), plus resistance to any fluoroquinolone and at least one of the three injectable second-line drugs (Amikacin, Kanamycin, or Capreomycin). Since Rifampicin resistance is a prerequisite for the diagnosis of XDR-TB, it is inherently ineffective and **not used** in its treatment. ### **Analysis of Incorrect Options** * **B. Isoniazid:** While XDR-TB is resistant to standard doses of Isoniazid, **High-dose Isoniazid** is sometimes included in WHO-recommended shorter regimens for drug-resistant TB if specific mutations (like *inhA* promoter mutations) suggest low-level resistance that can be overcome. * **C. Moxifloxacin:** This is a later-generation fluoroquinolone. While XDR-TB is resistant to "a" fluoroquinolone, newer agents like **high-dose Moxifloxacin** or Gatifloxacin were historically part of salvage regimens, though BPaL (Bedaquiline, Pretomanid, Linezolid) is now preferred. * **D. Capreomycin:** This is a cyclic peptide (injectable second-line drug). It was a mainstay in the treatment of MDR-TB and used in XDR-TB cases where sensitivity was still preserved to at least one injectable agent. ### **NEET-PG High-Yield Pearls** * **MDR-TB:** Resistance to Isoniazid (H) + Rifampicin (R). * **Pre-XDR-TB:** MDR-TB + resistance to any Fluoroquinolone. * **XDR-TB (New WHO Definition):** MDR-TB + resistance to any Fluoroquinolone + resistance to at least one Group A drug (**Bedaquiline or Linezolid**). * **Drug of Choice for XDR-TB:** The **BPaL regimen** (Bedaquiline, Pretomanid, and Linezolid) is currently the standard of care.

Question 913: In antiretroviral therapy, which of the following drugs should not be combined with Zidovudine?

A. Lamivudine
B. Nevirapine
C. Didanosine
D. Stavudine (Correct Answer)

Explanation: ### Explanation The correct answer is **Stavudine (D)**. **Why Stavudine is the correct answer:** Both **Zidovudine (AZT)** and **Stavudine (d4T)** are Nucleoside Reverse Transcriptase Inhibitors (NRTIs) that act as **thymidine analogs**. To become active, they must be phosphorylated by the same intracellular enzyme, **thymidine kinase**. When administered together, they compete for this enzyme. Zidovudine has a higher affinity for thymidine kinase and effectively inhibits the phosphorylation (activation) of Stavudine. This results in a **pharmacodynamic antagonism**, rendering the combination clinically ineffective and increasing the risk of toxicity without therapeutic benefit. **Analysis of Incorrect Options:** * **A. Lamivudine (3TC):** This is a cytosine analog. It uses different phosphorylation pathways and is the most common partner for Zidovudine (e.g., the Combivir regimen) due to synergistic effects. * **B. Nevirapine:** This is a Non-Nucleoside Reverse Transcriptase Inhibitor (NNRTI). It has a completely different mechanism of action (binding directly to the enzyme) and is frequently combined with Zidovudine in NNRTI-based regimens. * **C. Didanosine (ddI):** This is a purine (adenosine) analog. It does not compete with Zidovudine for activation and was historically used in dual NRTI combinations. **High-Yield Clinical Pearls for NEET-PG:** * **Zidovudine (AZT) Side Effects:** Macrocytic anemia (most common) and bone marrow suppression. It is the drug of choice for preventing mother-to-child transmission (MTCT) during labor. * **Stavudine (d4T) Side Effects:** Highest risk of peripheral neuropathy and lipodystrophy among NRTIs. * **Mnemonic:** Remember **"Z"** and **"S"** (Zidovudine and Stavudine) are both **Thymidine** analogs—they "Stay" away from each other to avoid competition.

Question 914: All of the following are useful in the management of severe Clostridium difficile infection, except?

A. Oral vancomycin
B. Neomycin enema (Correct Answer)
C. IV metronidazole
D. Tigecycline

Explanation: **Explanation:** The management of *Clostridioides difficile* infection (CDI) focuses on agents that reach therapeutic concentrations in the colon and possess activity against anaerobic gram-positive bacilli. **Why Neomycin is the correct answer (Except):** Neomycin is an aminoglycoside. Aminoglycosides are strictly aerobic in their mechanism of action (requiring oxygen for uptake into the bacterial cell) and are **ineffective against anaerobes** like *C. difficile*. Furthermore, neomycin is highly nephrotoxic and ototoxic if absorbed, and its use is generally limited to bowel preparation or hepatic encephalopathy. It has no role in treating CDI. **Analysis of other options:** * **Oral Vancomycin:** This is the **first-line treatment** for severe CDI. It is not absorbed systemically, ensuring high intraluminal concentrations directly at the site of infection. * **IV Metronidazole:** While oral metronidazole is used for mild cases, IV metronidazole is added to oral vancomycin in **fulminant/complicated** cases (e.g., ileus or shock) because it reaches the inflamed colon via biliary secretion and systemic circulation. * **Tigecycline:** This is a glycylcycline with broad-spectrum activity, including potent anti-anaerobic properties. It is considered a **rescue therapy** for refractory or severe CDI when standard treatments fail. **NEET-PG High-Yield Pearls:** 1. **Drug of Choice:** Oral Vancomycin or Fidaxomicin (Fidaxomicin has lower recurrence rates). 2. **Metronidazole:** Only antibiotic used **IV** for CDI (Vancomycin must be oral/rectal). 3. **Bezlotoxumab:** A monoclonal antibody against Toxin B used to prevent recurrence. 4. **Fecal Microbiota Transplant (FMT):** Indicated for multiple recurrences of CDI.

Question 915: Pyridoxine is given with which antitubercular drug?

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

Explanation: **Explanation:** **Correct Answer: B. Isoniazid (INH)** **Mechanism of Action & Rationale:** Isoniazid (INH) is structurally similar to **Pyridoxine (Vitamin B6)**. It promotes the excretion of pyridoxine and inhibits the enzyme **pyridoxine phosphokinase**, which converts pyridoxine to its active form, pyridoxal phosphate (PLP). PLP is a vital cofactor for the synthesis of neurotransmitters (like GABA). A deficiency in PLP leads to **peripheral neuropathy**, characterized by numbness and paresthesia in a "glove and stocking" distribution. To prevent this, pyridoxine is co-administered with INH, typically at a dose of 10–50 mg/day. **Analysis of Incorrect Options:** * **A. Rifampicin:** Its primary side effects are hepatotoxicity and the harmless orange-red discoloration of body fluids (urine, sweat, tears). It does not interfere with B6 metabolism. * **C. Pyrazinamide:** Known for causing hyperuricemia (leading to gout) and hepatotoxicity. It does not cause peripheral neuropathy. * **D. Ethambutol:** Its most characteristic side effect is **optic neuritis** (diminished visual acuity and red-green color blindness). It is not associated with B6 deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Risk Groups:** While B6 is not always mandatory for all patients, it is **essential** in those predisposed to neuropathy: diabetics, alcoholics, pregnant/lactating women, and malnourished patients. * **Sideroblastic Anemia:** INH can also cause microcytic anemia because PLP is a cofactor for ALA synthase (the rate-limiting step in heme synthesis). * **Metabolism:** INH is metabolized by **Acetylation**. "Slow acetylators" are at a significantly higher risk of developing peripheral neuropathy.

Question 916: What is the drug of choice for non-gonococcal urethritis?

A. Doxycycline (Correct Answer)
B. Tetracycline
C. Ciprofloxacin
D. Ceftriaxone

Explanation: **Explanation:** **Non-gonococcal urethritis (NGU)** is most commonly caused by *Chlamydia trachomatis*, followed by *Mycoplasma genitalium*. 1. **Why Doxycycline is correct:** According to the latest CDC and WHO guidelines, **Doxycycline (100 mg twice daily for 7 days)** is the first-line drug of choice for NGU. It is a bacteriostatic tetracycline that inhibits protein synthesis by binding to the 30S ribosomal subunit. It is highly effective against intracellular organisms like *Chlamydia*. While Azithromycin (1g single dose) was previously preferred for compliance, Doxycycline is now favored due to superior efficacy against *Mycoplasma genitalium* and rectal chlamydial infections. 2. **Why the other options are incorrect:** * **Tetracycline:** While in the same class, it requires four-times-daily dosing and has more gastrointestinal side effects and lower bioavailability compared to Doxycycline. * **Ciprofloxacin:** This is a fluoroquinolone. While it has some activity against Gram-negative bacteria, it is not the primary choice for NGU due to rising resistance and inferior efficacy against *Chlamydia* compared to tetracyclines. * **Ceftriaxone:** This is a third-generation cephalosporin and the **drug of choice for Gonococcal urethritis** (Gonorrhea). It is ineffective against *Chlamydia* because *Chlamydia* lacks a traditional peptidoglycan cell wall. **High-Yield Clinical Pearls for NEET-PG:** * **Syndromic Management (Kit 1):** In India, under the NACO guidelines, the Grey Kit (Azithromycin 1g + Cefixime 400mg) is used for the syndromic management of urethral discharge to cover both Gonorrhea and NGU. * **Pregnancy:** Doxycycline is contraindicated in pregnancy (teratogenic - causes bone/teeth discoloration). In pregnant patients with NGU, **Azithromycin** is the drug of choice. * **Co-infection:** Always treat for both Gonorrhea and Chlamydia if the specific pathogen is not identified.

Question 917: Which antimicrobial drug is used orally exclusively for urinary tract infections or bacterial diarrheas?

A. Bacampicillin
B. Nalidixic acid (Correct Answer)
C. Azithromycin
D. Pefloxacin

Explanation: **Explanation:** **Nalidixic acid** is the correct answer because it is a first-generation quinolone with a unique pharmacokinetic profile. When taken orally, it is rapidly absorbed but undergoes extensive metabolism and rapid renal excretion. Consequently, it fails to achieve therapeutic systemic concentrations in the blood or tissues but reaches **high bactericidal concentrations in the urine**. Additionally, its unabsorbed portion remains active in the gut. Therefore, its clinical utility is strictly limited to **uncomplicated Urinary Tract Infections (UTIs)** and **bacillary dysentery (bacterial diarrhea)**. **Analysis of Incorrect Options:** * **A. Bacampicillin:** This is a prodrug of Ampicillin (a penicillin). It achieves high systemic levels and is used for various infections, including respiratory tract infections and otitis media, not just UTIs. * **C. Azithromycin:** A macrolide with extensive tissue distribution and a long half-life. It is used systemically for pneumonia, typhoid, and STDs (Chlamydia). * **D. Pefloxacin:** A second-generation fluoroquinolone. Unlike nalidixic acid, it achieves excellent systemic distribution and can cross the blood-brain barrier; it is used for systemic infections like enteric fever. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Nalidixic acid inhibits **DNA Gyrase** (Topoisomerase II), preventing bacterial DNA replication. * **Resistance:** Resistance develops rapidly during treatment; hence, it has been largely replaced by newer fluoroquinolones (e.g., Ciprofloxacin). * **Contraindication:** It should be avoided in patients with **G6PD deficiency** as it can precipitate hemolysis. * **Side Effects:** It is known to cause visual disturbances and can increase intracranial pressure (pseudotumor cerebri) in children.

Question 918: Which of the following anti-retroviral drugs does NOT cause peripheral neuropathy?

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

Explanation: **Explanation:** The primary mechanism behind Nucleoside Reverse Transcriptase Inhibitor (NRTI)-induced peripheral neuropathy is **mitochondrial toxicity**. This occurs because certain NRTIs inhibit **DNA polymerase-gamma**, the enzyme responsible for mitochondrial DNA replication. **1. Why Lamivudine (3TC) is the correct answer:** Lamivudine has a very low affinity for mitochondrial DNA polymerase-gamma. Consequently, it is considered one of the least toxic NRTIs and does **not** typically cause peripheral neuropathy or significant lactic acidosis. **2. Analysis of Incorrect Options:** * **Didanosine (ddI) and Zalcitabine (ddC):** These are the most potent inhibitors of DNA polymerase-gamma. They are notorious for causing dose-limiting **"stocking-and-glove" peripheral neuropathy** and pancreatitis. (Note: Zalcitabine is now largely obsolete due to this toxicity). * **Zidovudine (AZT):** While Zidovudine is more commonly associated with **bone marrow suppression** (anemia/neutropenia) and **myopathy**, it still possesses higher mitochondrial toxicity compared to Lamivudine. However, in the context of this classic "D-drug" comparison, Lamivudine is the definitive answer as it lacks this side effect entirely. **Clinical Pearls for NEET-PG:** * **The "D" Drugs:** Remember that **D**idanosine, **D**eoxycytidine (Zalcitabine), and Stavudine (**d**4T) are the primary culprits for peripheral neuropathy and lipoatrophy. * **Zidovudine (AZT):** High-yield side effect is **Megaloblastic Anemia**. * **Abacavir:** Associated with a life-threatening **Hypersensitivity Reaction** linked to the **HLA-B*5701** allele. * **Tenofovir:** Known for causing **Fanconi Syndrome** and decreased bone mineral density.

Question 919: Following an appendectomy, a 28-year-old man is placed on ceftizoxime sodium. This antibiotic is unlikely to be effective against which of the following?

A. Pseudomonas (Correct Answer)
B. Staphylococcus aureus
C. Neisseria gonorrhoeae
D. Bacteroides fragilis

Explanation: **Explanation:** Ceftizoxime is a **third-generation cephalosporin**. To answer this question correctly, one must understand the specific spectrum of activity within the third-generation class, particularly the distinction between those that cover *Pseudomonas* and those that do not. **Why Pseudomonas is the Correct Answer:** While third-generation cephalosporins have expanded Gram-negative coverage compared to earlier generations, **Ceftizoxime lacks activity against *Pseudomonas aeruginosa***. In the third-generation class, only **Ceftazidime** and **Cefoperazone** possess reliable anti-pseudomonal activity. **Analysis of Incorrect Options:** * **Staphylococcus aureus:** Third-generation cephalosporins maintain some activity against methicillin-susceptible *S. aureus* (MSSA), though they are generally less potent than first-generation agents (like Cefazolin). * **Neisseria gonorrhoeae:** This class is highly effective against Gram-negative cocci. Ceftriaxone is the drug of choice, but Ceftizoxime also maintains excellent activity against *Neisseria* species. * **Bacteroides fragilis:** Ceftizoxime is unique among many third-generation cephalosporins because it possesses notable **anaerobic activity**, making it effective against *B. fragilis*. This is why it is often used in intra-abdominal infections like appendicitis. **High-Yield NEET-PG Pearls:** 1. **Anti-pseudomonal Cephalosporins:** Remember the "3rd and 4th" rule: **Ceftazidime, Cefoperazone** (3rd gen), and **Cefepime** (4th gen). 2. **Anaerobic Coverage:** Most cephalosporins lack anaerobic coverage. Exceptions include **Cefoxitin, Cefotetan** (2nd gen), and **Ceftizoxime** (3rd gen). 3. **Excretion:** Unlike Ceftriaxone (biliary excretion), Ceftizoxime is primarily excreted by the kidneys.

Question 920: A 30-year-old pregnant woman develops tuberculosis. Which of the following antitubercular drugs should be avoided during pregnancy?

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

Explanation: ### Explanation **Correct Option: C. Streptomycin** **Why it is correct:** Streptomycin is an aminoglycoside that is strictly contraindicated during pregnancy (FDA Category D). It crosses the placental barrier and is known to be **ototoxic** to the fetus. Exposure during gestation can lead to permanent **congenital deafness** (damage to the 8th cranial nerve) and potential nephrotoxicity in the developing fetus. **Why the other options are incorrect:** * **A. Isoniazid (INH):** Considered safe in pregnancy. However, because pregnancy increases the risk of peripheral neuropathy, it must always be co-administered with **Pyridoxine (Vitamin B6)**. * **B. Rifampicin:** Considered safe and is a core component of the RNTCP/WHO regimen for pregnant women. It may rarely cause neonatal hypoprothrombinemia, so Vitamin K prophylaxis is recommended at birth. * **D. Ethambutol:** Considered the safest of the first-line drugs in pregnancy with no documented teratogenic effects. **NEET-PG High-Yield Pearls:** 1. **Standard Regimen:** The WHO and National Guidelines recommend the standard 2HRZE/4HRE regimen for pregnant women. Only Streptomycin is excluded from the first-line list. 2. **Pyrazinamide (PZA):** While some older texts debated its safety, the WHO and Indian national guidelines now include PZA in the intensive phase for pregnant patients. 3. **Second-line Drugs to Avoid:** Beyond Streptomycin, other aminoglycosides (Kanamycin, Amikacin) and **Fluoroquinolones** (due to cartilage damage) should generally be avoided unless the benefit outweighs the risk (e.g., MDR-TB). 4. **Breastfeeding:** All first-line antitubercular drugs are compatible with breastfeeding as they are excreted in negligible amounts in breast milk.

Practice by Chapter

Beta-Lactam Antibiotics

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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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