Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 991: Reverse transcriptase inhibitors are indicated for which of the following viral infections?

A. HIV (Human Immunodeficiency Virus) (Correct Answer)
B. HBV (Hepatitis B Virus)
C. HCV (Hepatitis C Virus)
D. All of the above

Explanation: **Explanation:** **1. Why HIV is the correct answer:** Reverse Transcriptase Inhibitors (RTIs) are the cornerstone of Highly Active Antiretroviral Therapy (HAART) for **HIV**. HIV is a retrovirus that uses the enzyme **Reverse Transcriptase** to convert its single-stranded RNA genome into double-stranded DNA, which is then integrated into the host cell genome. RTIs are classified into two groups: * **NRTIs (Nucleoside/Nucleotide RTIs):** e.g., Zidovudine, Tenofovir, Emtricitabine. They act as chain terminators. * **NNRTIs (Non-Nucleoside RTIs):** e.g., Efavirenz, Nevirapine. They bind directly to the enzyme to inhibit its activity. **2. Why other options are incorrect:** * **HBV (Hepatitis B Virus):** While HBV is a DNA virus that *does* use a reverse transcription step in its replication cycle, and drugs like **Tenofovir and Lamivudine** are used to treat it, the term "Reverse Transcriptase Inhibitors" as a pharmacological class is classically and primarily associated with HIV management in standard textbooks (KDT, Katzung). * **HCV (Hepatitis C Virus):** HCV is a positive-sense RNA virus that replicates using **RNA-dependent RNA polymerase (NS5B)**. It does not utilize reverse transcription. Treatment involves Direct-Acting Antivirals (DAAs) like Sofosbuvir and Daclatasvir. * **All of the above:** Incorrect because HCV does not utilize reverse transcriptase. **High-Yield Clinical Pearls for NEET-PG:** * **Zidovudine (AZT):** Associated with bone marrow suppression (anemia/neutropenia) and is used for preventing vertical transmission. * **Tenofovir:** A nucleotide analog (NtRTI) often associated with renal toxicity (Fanconi syndrome) and decreased bone mineral density. * **Abacavir:** Requires screening for **HLA-B*5701** to prevent life-threatening hypersensitivity reactions. * **Nevirapine:** Known for causing severe skin rashes (Stevens-Johnson Syndrome) and hepatotoxicity.

Question 992: Peripheral neuritis is a complication of which drug?

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

Explanation: **Explanation:** **Isoniazid (INH)** is the correct answer because it interferes with the metabolism of **Pyridoxine (Vitamin B6)**. INH is chemically similar to pyridoxine and promotes its excretion in the urine. Additionally, it inhibits the enzyme *pyridoxine phosphokinase*, which converts B6 into its active form (pyridoxal phosphate). This deficiency leads to **peripheral neuropathy**, characterized by "glove and stocking" paresthesia [1]. This side effect is more common in "slow acetylators" and patients with pre-existing risks like diabetes, alcoholism, or malnutrition [1]. **Analysis of Incorrect Options:** * **Ethambutol:** Its most characteristic side effect is **Retrobulbar Neuritis**, which presents as decreased visual acuity and red-green color blindness. It does not typically cause peripheral nerve damage. * **Rifampicin:** Known for causing **hepatotoxicity** and a harmless **orange-red discoloration** of body fluids (urine, sweat, tears). It can also cause a "flu-like syndrome" if taken intermittently [4]. * **Pyrazinamide:** The most common side effects are **hyperuricemia** (leading to gout) and hepatotoxicity [3]. It is the most hepatotoxic drug among the first-line ATT. **High-Yield Clinical Pearls for NEET-PG:** * **Prevention:** Peripheral neuritis can be prevented by co-administering **Prophylactic Pyridoxine (10 mg/day)** [1]. * **Treatment:** If neuritis has already developed, the dose of Pyridoxine is increased to **100 mg/day**. * **Metabolism:** INH is metabolized by **Acetylation** (Phase II reaction) [2]. Slow acetylators are at higher risk for neuropathy, while fast acetylators are at higher risk for hepatotoxicity.

Question 993: Which tissue schizontocide prevents relapse of vivax malaria?

A. Quinine
B. Primaquine (Correct Answer)
C. Pyrimethamine
D. Chloroquine

Explanation: **Explanation:** The correct answer is **Primaquine**. **1. Why Primaquine is correct:** *Plasmodium vivax* and *Plasmodium ovale* have a unique life cycle stage called **hypnozoites**—dormant forms that remain in the liver (exoerythrocytic stage). These hypnozoites can "wake up" months or years later, causing a clinical **relapse**. Primaquine is the prototype **tissue schizontocide** that effectively kills these hepatic stages. It is the only drug (along with the newer Tafenoquine) used for the **radical cure** of vivax malaria to prevent recurrence. **2. Why other options are incorrect:** * **Chloroquine:** It is a potent **blood schizontocide**. While it is the drug of choice for treating the acute erythrocytic phase of malaria, it has no effect on the latent liver stages (hypnozoites). * **Quinine:** Another blood schizontocide used primarily for severe or chloroquine-resistant malaria. Like chloroquine, it cannot prevent relapses. * **Pyrimethamine:** This is a folate antagonist (DHFR inhibitor) used as a **slow-acting blood schizontocide** and a sporontocide. It does not eliminate hypnozoites. **3. High-Yield NEET-PG Pearls:** * **G6PD Deficiency:** Before prescribing Primaquine, patients **must** be screened for G6PD deficiency. The drug causes oxidative stress, leading to **acute hemolysis** in deficient individuals. * **Pregnancy:** Primaquine is **contraindicated** in pregnancy because the fetus is relatively G6PD deficient. * **Gametocidal Action:** Primaquine is also highly effective against the gametocytes of all species, including *P. falciparum*, thereby preventing the transmission of malaria to mosquitoes. * **Tafenoquine:** A long-acting analog of Primaquine that can be given as a single dose for radical cure.

Question 994: Which drug interferes with pyridoxine metabolism?

A. Isoniazid (INH) (Correct Answer)
B. Tetracycline
C. Erythromycin
D. Rifampicin

Explanation: **Isoniazid (INH)** is the correct answer because it is a structural analogue of **Pyridoxine (Vitamin B6)**. It interferes with B6 metabolism through two primary mechanisms:1. It inhibits the enzyme **pyridoxine phosphokinase**, which converts pyridoxine into its active form, pyridoxal-5-phosphate (PLP).2. It reacts with PLP to form **isoniazid-pyridoxal hydrazones**, which are rapidly excreted in the urine.Since PLP is a vital cofactor for the synthesis of neurotransmitters like GABA, its deficiency leads to **peripheral neuropathy** (the most common side effect) [1] and, in rare cases, CNS toxicity/seizures [1].**Analysis of Incorrect Options:** * **Tetracycline:** Acts by inhibiting the 30S ribosomal subunit. Its major side effects involve bone/teeth discoloration and GI upset, not vitamin interference.* **Erythromycin:** A macrolide that inhibits the 50S ribosomal subunit. It is known for causing motilin-like GI effects and cholestatic jaundice.* **Rifampicin:** Inhibits DNA-dependent RNA polymerase. Its high-yield side effects include orange-colored body fluids and potent induction of Cytochrome P450 enzymes.**High-Yield Clinical Pearls for NEET-PG:** * **Prophylaxis:** To prevent neuropathy, INH is co-administered with **10–50 mg/day of Pyridoxine** [1].* **Risk Groups:** Malnourished individuals, alcoholics, diabetics [1], and **slow acetylators** (who have higher plasma levels of INH) [1] are at increased risk.* **Acute Toxicity:** In cases of INH overdose (presenting with seizures), intravenous Pyridoxine is the specific antidote.* **Sideroblastic Anemia:** INH can also cause this because PLP is a cofactor for ALA synthase, the rate-limiting enzyme in heme synthesis.

Question 995: Which of the following antimicrobials shows time-dependent killing?

A. Ofloxacin
B. Amikacin
C. Penicillin (Correct Answer)
D. Ciprofloxacin

Explanation: ### Explanation **1. Why Penicillin is Correct (The Underlying Concept)** Antimicrobial killing patterns are categorized based on the relationship between drug concentration and efficacy. **Penicillin** (and all Beta-lactams) exhibits **Time-dependent killing**. For these drugs, the clinical efficacy is best predicted by the **T > MIC** (the duration of time the free drug concentration remains above the Minimum Inhibitory Concentration). Increasing the concentration far above the MIC does not significantly increase the rate or extent of bacterial killing; instead, maintaining a steady level above the MIC is the goal. This is why these drugs are often administered via frequent dosing or continuous infusion. **2. Why the Other Options are Incorrect** * **Ofloxacin & Ciprofloxacin (Fluoroquinolones):** These exhibit **Concentration-dependent killing**. Their efficacy depends on achieving a high peak concentration relative to the MIC (Cmax/MIC ratio). * **Amikacin (Aminoglycosides):** These also show **Concentration-dependent killing**. They are characterized by a significant **Post-Antibiotic Effect (PAE)**, allowing for once-daily "pulse" dosing despite short half-lives. **3. High-Yield Clinical Pearls for NEET-PG** * **Time-Dependent Drugs:** Remember the mnemonic **"B-L-V"** (Beta-lactams, Linezolid, Vancomycin). *Note: Vancomycin is increasingly categorized by AUC/MIC, but traditionally grouped here for exams.* * **Concentration-Dependent Drugs:** Remember **"A-F"** (Aminoglycosides, Fluoroquinolones, Daptomycin, Metronidazole). * **Post-Antibiotic Effect (PAE):** This is the persistent suppression of bacterial growth after the drug concentration falls below the MIC. It is most prominent with Aminoglycosides (against Gram-negatives) and Fluoroquinolones. * **Dosing Strategy:** For time-dependent drugs, the goal is **frequent dosing**; for concentration-dependent drugs, the goal is **high-dose, infrequent administration**.

Question 996: Which of the following drugs is not effective by the oral route?

A. Erythromycin
B. Ciprofloxacin
C. Streptomycin (Correct Answer)
D. Albendazole

Explanation: **Explanation:** The correct answer is **Streptomycin**. **1. Why Streptomycin is the correct answer:** Streptomycin belongs to the **Aminoglycoside** class of antibiotics. Aminoglycosides are highly polar, polycationic compounds. Due to their high degree of ionization at physiological pH, they are **not absorbed from the gastrointestinal tract** (GIT) to any significant extent. If administered orally, they remain in the gut lumen and are excreted unchanged in the feces. Therefore, for systemic infections (like Tuberculosis or Plague), Streptomycin must be administered via the **intramuscular (IM)** route. **2. Why the other options are incorrect:** * **Erythromycin (Macrolide):** It is acid-labile but is formulated as enteric-coated tablets or stable esters (e.g., Erythromycin stearate) to allow effective oral absorption. * **Ciprofloxacin (Fluoroquinolone):** It has excellent oral bioavailability (approx. 70-80%) and is commonly used as an oral agent for UTIs and respiratory infections. * **Albendazole (Anthelmintic):** It is specifically designed for oral administration. It acts locally in the gut for intraluminal parasites and is absorbed (enhanced by a fatty meal) for systemic effects against hydatid disease or neurocysticercosis. **3. NEET-PG High-Yield Pearls:** * **Aminoglycoside Exception:** Neomycin and Kanamycin are sometimes given orally, but *only* for local action (e.g., hepatic coma to kill ammonia-producing bacteria or bowel preparation before surgery), never for systemic infections. * **Oral Bioavailability:** Drugs with poor oral absorption (like Aminoglycosides and Vancomycin) are often used orally only when the target site is the bowel lumen (e.g., oral Vancomycin for *C. difficile* colitis). * **Streptomycin Side Effects:** Remember the "8th Cranial Nerve" toxicity (vestibulotoxicity) and its contraindication in pregnancy due to fetal ototoxicity.

Question 997: Once weekly administration of which of the following antibiotics has prophylactic activity against bacteraemia caused by M.avium complex in AIDS patients?

A. Azithromycin (Correct Answer)
B. Clarithromycin
C. Isoniazid
D. Rifabutin

Explanation: **Explanation:**The primary goal of prophylaxis against **Mycobacterium avium complex (MAC)** in HIV/AIDS patients (typically when CD4 counts fall below 50 cells/mm³) is to prevent disseminated infection. **1. Why Azithromycin is correct:**Azithromycin is a macrolide with a exceptionally long tissue half-life (approx. 68 hours). This pharmacokinetic property allows for once-weekly dosing (1200 mg) [1], which significantly improves patient compliance. It is the preferred agent for primary prophylaxis of MAC because it is effective, well-tolerated, and requires infrequent administration. **2. Why the other options are incorrect:** * **Clarithromycin:** While also a first-line agent for MAC, it has a much shorter half-life than azithromycin. It requires **twice-daily (BID)** administration for prophylaxis, making it less convenient than weekly azithromycin. * **Isoniazid:** This is the mainstay for *Mycobacterium tuberculosis* (TB) prophylaxis (Latent TB Infection), but it has no clinical activity against MAC. * **Rifabutin:** This is considered an alternative for MAC prophylaxis if macrolides are not tolerated. However, it must be administered **daily**, carries a risk of uveitis, and involves significant drug-drug interactions via CYP450 induction. **Clinical Pearls for NEET-PG:** * **Initiation Criteria:** MAC prophylaxis is indicated in HIV patients with **CD4 < 50 cells/mm³**. * **Treatment of Active MAC:** Unlike prophylaxis (monotherapy), active MAC infection requires **combination therapy** (usually Clarithromycin + Ethambutol ± Rifabutin) to prevent resistance. * **Discontinuation:** Prophylaxis can be stopped if the CD4 count remains **>100 cells/mm³ for at least 3 months** in response to ART.

Question 998: Which of the following drug toxicities can be seen in Crigler-Najjar syndrome?

A. Tenofovir
B. Zidovudine
C. Atazanavir (Correct Answer)
D. Nevirapine

Explanation: **Explanation:** The correct answer is **Atazanavir**. **1. Why Atazanavir is correct:** Atazanavir is a Protease Inhibitor (PI) used in HIV treatment. Its primary metabolic pathway involves inhibition of the enzyme **UGT1A1 (UDP-glucuronosyltransferase)**. This enzyme is responsible for the conjugation of bilirubin in the liver. * **Crigler-Najjar Syndrome** is a genetic disorder characterized by a deficiency (Type II) or total absence (Type I) of UGT1A1. * When a patient with Crigler-Najjar syndrome (or the milder Gilbert syndrome) takes Atazanavir, the drug further inhibits the already deficient enzyme, leading to a significant rise in unconjugated bilirubin and clinical **jaundice/hyperbilirubinemia**. **2. Why the other options are incorrect:** * **Tenofovir (A):** A Nucleotide Reverse Transcriptase Inhibitor (NRTI) primarily associated with **nephrotoxicity** (Fanconi syndrome) and bone mineral density loss, not UGT inhibition. * **Zidovudine (B):** An NRTI known for **bone marrow suppression** (anemia, neutropenia) and mitochondrial toxicity (lactic acidosis). * **Nevirapine (D):** A Non-Nucleoside Reverse Transcriptase Inhibitor (NNRTI) associated with severe **hepatotoxicity** and skin rashes (Stevens-Johnson Syndrome), but it does not specifically target the UGT1A1 pathway like Atazanavir. **Clinical Pearls for NEET-PG:** * **Atazanavir** is often called the "bananavir" in medical mnemonics because it causes yellowing of the skin (jaundice). * **Gilbert Syndrome:** Similar to Crigler-Najjar but milder; Atazanavir is also contraindicated or used with extreme caution here. * **Indinavir:** Another Protease Inhibitor that can cause hyperbilirubinemia and nephrolithiasis (kidney stones).

Question 999: What is the drug of choice for prophylaxis of tuberculosis?

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

Explanation: **Explanation:** **Isoniazid (INH)** is the drug of choice for the prophylaxis of tuberculosis (Latent TB Infection) [1]. The primary goal of prophylaxis is to eliminate the dormant bacilli in individuals who have been infected but do not have active disease, thereby preventing progression to clinical TB [1]. * **Why Isoniazid is the Correct Answer:** INH is highly bactericidal against rapidly dividing mycobacteria and remains effective against dormant bacilli during their occasional bursts of metabolic activity. It is preferred due to its high efficacy, low cost, and established safety profile in prophylactic regimens (typically administered for 6 to 9 months) [1]. **Analysis of Incorrect Options:** * **Rifampicin (Option A):** While Rifampicin is a potent bactericidal drug used in the primary treatment of TB and can be used for prophylaxis in cases of INH resistance or intolerance, it is not the first-line "drug of choice" for standard prophylaxis. * **Pyrazinamide (Option C):** This drug is primarily used for its "sterilizing" effect in the first two months of active TB treatment to kill intracellular bacilli. It is not used alone for prophylaxis due to the high risk of hepatotoxicity. * **Streptomycin (Option D):** An aminoglycoside that must be administered via intramuscular injection. Its toxicity profile (ototoxicity and nephrotoxicity) and route of administration make it unsuitable for long-term prophylaxis. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** INH inhibits the synthesis of **mycolic acids**, essential components of the mycobacterial cell wall, by targeting the enzyme *InhA*. * **Metabolism:** INH is metabolized via **Acetylation**. "Slow acetylators" are at a higher risk of peripheral neuropathy. * **Side Effects:** The most common side effect is **peripheral neuropathy** (prevented by co-administering **Vitamin B6/Pyridoxine**) and the most serious is **hepatotoxicity**. * **Chemoprophylaxis Indications:** Commonly indicated for household contacts of open cases (especially children <6 years) and HIV-positive individuals with a positive TST/IGRA [1].

Question 1000: Which of the following metabolic disturbances is associated with the use of Amphotericin B?

A. Hypokalemia (Correct Answer)
B. Hyperkalemia
C. Hypermagnesemia
D. Hyponatremia

Explanation: **Explanation:** **Amphotericin B** is a potent antifungal that acts by binding to ergosterol in fungal cell membranes, creating pores. However, it also binds to cholesterol in human renal tubular cells, leading to significant nephrotoxicity. **Why Hypokalemia is correct:** The primary mechanism for metabolic disturbances is **Renal Tubular Acidosis (Type 1/Distal RTA)**. Amphotericin B increases the permeability of the distal tubule and collecting duct membranes. This leads to a "leak" of intracellular cations into the tubular lumen. Specifically, it promotes the wasting of **Potassium (K⁺)** and **Magnesium (Mg²⁺)**. Therefore, **Hypokalemia** and **Hypomagnesemia** are classic side effects. **Analysis of Incorrect Options:** * **B. Hyperkalemia:** Incorrect. Amphotericin B causes potassium wasting, not retention. Hyperkalemia is generally seen in renal failure where GFR is severely compromised, but the specific tubular effect of Amphotericin B is potassium loss. * **C. Hypermagnesemia:** Incorrect. Similar to potassium, magnesium is lost through the damaged tubules, leading to **Hypomagnesemia**. * **D. Hyponatremia:** Incorrect. While electrolyte shifts occur, the hallmark disturbances are related to K⁺ and Mg²⁺. Sodium levels are not the primary diagnostic metabolic disturbance associated with this drug. **High-Yield Clinical Pearls for NEET-PG:** * **Pre-medication:** To reduce infusion-related reactions (fever, chills), patients are often given NSAIDs or Hydrocortisone. * **Saline Loading:** Administering 1 liter of normal saline before the infusion can significantly reduce the risk of nephrotoxicity. * **Liposomal Amphotericin B:** This formulation is preferred as it has lower nephrotoxicity because it delivers the drug more specifically to fungal cells and less to renal tubules. * **Anemia:** It can also cause normocytic normochromic anemia due to decreased Erythropoietin production.

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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