Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 591: Which of the following antitubercular drugs has maximum cerebrospinal fluid (CSF) penetration?

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

Explanation: **Explanation:** The penetration of antitubercular drugs into the cerebrospinal fluid (CSF) is a critical factor in managing Tuberculous Meningitis (TBM). **Why Isoniazid (INH) is Correct:** Isoniazid is a small, water-soluble molecule with low protein binding. It achieves excellent penetration into the CSF, reaching concentrations nearly **equal to those in the plasma** (approx. 90–100%), regardless of whether the meninges are inflamed or intact. This makes it the backbone of TBM treatment. Pyrazinamide (not listed here) also shares this property of excellent CSF penetration. **Analysis of Incorrect Options:** * **Streptomycin (Option A):** As an aminoglycoside, it is a large, polar molecule. It penetrates the blood-brain barrier (BBB) very poorly, even when meninges are inflamed. It is generally avoided in TBM unless resistance dictates otherwise. * **Rifampicin (Option C):** It is a large, lipid-soluble molecule but is highly protein-bound. It achieves only about 5–25% of plasma concentration in the CSF. While essential for treatment, its penetration is significantly lower than INH. * **Ethambutol (Option D):** It has poor CSF penetration (approx. 10–20%) and only crosses the BBB in significant amounts when the meninges are acutely inflamed. **High-Yield NEET-PG Pearls:** * **Best CSF Penetration:** Isoniazid and Pyrazinamide (reach therapeutic levels even without inflammation). * **Moderate CSF Penetration:** Rifampicin and Ethambutol (penetrate better when meninges are inflamed). * **Poor CSF Penetration:** Streptomycin and other aminoglycosides. * **Drug of Choice for TBM:** The standard intensive phase (HRZE) is used, but doses of Isoniazid and Pyrazinamide are most crucial for CNS efficacy. Corticosteroids (Dexamethasone) are often added to reduce neurological complications.

Question 592: Which of the following is NOT a form of renal damage caused by amphotericin B?

A. Azotemia
B. Renal tubular acidosis
C. Glomerulonephritis (Correct Answer)
D. Hypokalemia

Explanation: **Explanation:** Amphotericin B is notorious for its dose-dependent nephrotoxicity, which occurs via two primary mechanisms: direct toxic effects on the renal tubular epithelium and drug-induced renal vasoconstriction leading to decreased renal blood flow. **Why Glomerulonephritis is the Correct Answer:** Glomerulonephritis is an inflammatory process typically mediated by immune complexes or antibodies (e.g., Post-streptococcal GN). Amphotericin B causes **tubular and vascular damage**, not an immunological inflammatory response in the glomeruli. While it reduces the Glomerular Filtration Rate (GFR) due to vasoconstriction, it does not cause "Glomerulonephritis." **Analysis of Incorrect Options:** * **Azotemia:** This is the most common manifestation. Amphotericin B causes constriction of the afferent arterioles, leading to decreased renal blood flow and a rise in serum creatinine and BUN. * **Renal Tubular Acidosis (RTA):** It increases the permeability of the distal tubular membrane, leading to a "leak" of hydrogen ions. This specifically results in **Type 1 (Distal) RTA**. * **Hypokalemia:** The increased tubular permeability also leads to significant wasting of potassium and magnesium. This electrolyte imbalance is a hallmark side effect requiring frequent monitoring and supplementation. **NEET-PG High-Yield Pearls:** * **Liposomal Amphotericin B:** Developed to reduce nephrotoxicity by targeting the drug to the reticuloendothelial system rather than the kidneys. * **Pre-loading:** Administering a **Normal Saline bolus (1 liter)** before the infusion ("saline loading") significantly reduces the risk of azotemia. * **Other Side Effects:** "Shake and bake" reaction (fever/chills) and normocytic normochromic anemia (due to decreased erythropoietin).

Question 593: Hemolysis in G6PD deficiency may be caused by all of the following, except:

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

Explanation: **Explanation:** The core concept behind drug-induced hemolysis in G6PD deficiency is **oxidative stress**. G6PD is the rate-limiting enzyme in the pentose phosphate pathway, responsible for producing NADPH. NADPH maintains a pool of reduced glutathione, which neutralizes reactive oxygen species (ROS). In G6PD-deficient individuals, exposure to oxidizing drugs leads to the oxidation of hemoglobin into **Heinz bodies**, causing splenic sequestration and acute hemolysis. **Why Pyrimethamine is the Correct Answer:** Pyrimethamine is a dihydrofolate reductase (DHFR) inhibitor. Unlike many other antimalarials, it does not possess significant oxidizing properties and is generally considered **safe** to use in patients with G6PD deficiency. It does not trigger the production of ROS that leads to red cell destruction. **Analysis of Incorrect Options:** * **Primaquine (Option A):** This is the most notorious trigger for hemolysis in G6PD deficiency. It is a potent oxidizing agent. Testing for G6PD levels is mandatory before starting radical cure for *P. vivax* with Primaquine. * **Chloroquine (Option B):** While the risk is significantly lower than with Primaquine, Chloroquine is a 4-aminoquinoline that can cause hemolysis in individuals with severe G6PD variants (like the Mediterranean type). * **Quinine (Option D):** Quinine is known to cause oxidative stress and is classically associated with "Blackwater Fever" (massive intravascular hemolysis), especially in the context of G6PD deficiency. **High-Yield Clinical Pearls for NEET-PG:** 1. **Other High-Yield Triggers:** Sulfonamides (Dapsone, Cotrimoxazole), Nitrofurantoin, Nalidixic acid, and Fava beans (Favism). 2. **Peripheral Smear Findings:** Look for **Heinz bodies** (denatured hemoglobin) and **Bite cells** (degmacytes) resulting from splenic macrophage action. 3. **Inheritance:** G6PD deficiency is an **X-linked recessive** disorder, primarily affecting males. 4. **Protective Benefit:** G6PD deficiency provides a selective evolutionary advantage by offering protection against *Plasmodium falciparum* malaria.

Question 594: Which of the following antiretroviral agents can cause bone marrow suppression?

A. Ritonavir
B. Zidovudine (Correct Answer)
C. Stavudine
D. Didanosine

Explanation: **Explanation:** **Zidovudine (AZT)**, a Nucleoside Reverse Transcriptase Inhibitor (NRTI), is notorious for causing **bone marrow suppression**, which manifests primarily as **macrocytic anemia** and **neutropenia**. The underlying mechanism involves the inhibition of mitochondrial DNA polymerase-gamma and the direct toxic effect on bone marrow progenitor cells. This side effect is dose-dependent and often requires monitoring of complete blood counts (CBC). **Analysis of Incorrect Options:** * **Ritonavir (Option A):** A Protease Inhibitor (PI) primarily known for causing gastrointestinal distress, perioral paresthesia, and metabolic complications (dyslipidemia, insulin resistance). It is most commonly used in low doses as a "pharmacokinetic booster" due to its potent inhibition of CYP3A4. * **Stavudine (Option C):** While an NRTI, its hallmark toxicity is **peripheral neuropathy** and lipodystrophy. It has a higher affinity for mitochondrial DNA polymerase than other NRTIs, leading to lactic acidosis. * **Didanosine (Option D):** This NRTI is classic for causing **pancreatitis** and peripheral neuropathy. It does not typically cause significant marrow suppression. **High-Yield Clinical Pearls for NEET-PG:** * **Zidovudine** is the drug of choice for preventing **vertical transmission** (mother-to-child) of HIV during pregnancy and labor. * If a patient on Zidovudine develops severe anemia, it is often managed by switching to **Tenofovir** or using Erythropoietin. * **Mnemonic for NRTI toxicities:** **Z**idovudine (**Z**ero blood cells/Anemia), **S**tavudine (**S**ensory neuropathy), **D**idanosine (**D**igestive/Pancreatitis), **A**bacavir (**A**llergy/Hypersensitivity).

Question 595: What is the drug of choice for Kala-azar?

A. Pentamidine
B. Liposomal amphotericin B (Correct Answer)
C. Sodium stibogluconate
D. Ketoconazole

Explanation: **Explanation:** **Kala-azar (Visceral Leishmaniasis)** is caused by *Leishmania donovani*. The current **Drug of Choice (DOC)**, as per WHO and National Vector Borne Disease Control Programme (NVBDCP) guidelines, is **Liposomal Amphotericin B (LAmB)**. 1. **Why Liposomal Amphotericin B is correct:** Amphotericin B acts by binding to ergosterol in the fungal/protozoal cell membrane, creating pores. The **liposomal formulation** is preferred because it specifically targets the Reticuloendothelial System (liver and spleen) where the *Leishmania* parasites reside. It offers high efficacy (>95% cure rate) with a single-dose infusion (10 mg/kg) and significantly lower nephrotoxicity compared to the conventional deoxycholate form. 2. **Why other options are incorrect:** * **Sodium Stibogluconate (SSG):** Historically the DOC, but now largely abandoned in the Indian subcontinent (especially Bihar) due to widespread **antimonial resistance** and severe cardiotoxicity (QT prolongation). * **Pentamidine:** Previously used as a second-line agent, but its use is limited by severe side effects like insulin-dependent diabetes mellitus, hypotension, and renal failure. * **Ketoconazole:** An oral antifungal that has some activity against cutaneous leishmaniasis but is **ineffective** for the visceral form (Kala-azar). **High-Yield Clinical Pearls for NEET-PG:** * **Miltefosine:** The only **oral** drug for Kala-azar; however, it is teratogenic (contraindicated in pregnancy). * **Post-Kala-azar Dermal Leishmaniasis (PKDL):** The DOC is also Liposomal Amphotericin B (though treatment duration is longer). * **Vector:** Transmitted by the bite of the female sandfly (*Phlebotomus argentipes*). * **Diagnosis:** The gold standard is the demonstration of **LD bodies** in splenic or bone marrow aspirates; **rK39** is the rapid diagnostic test of choice.

Question 596: Cotrimoxazole is the drug of choice for:

A. Actinomyces
B. Nocardia (Correct Answer)
C. Both Actinomyces and Nocardia
D. None of the above

Explanation: **Explanation:** **Cotrimoxazole** (a fixed-dose combination of Sulfamethoxazole and Trimethoprim) is the **drug of choice for Nocardiosis** caused by *Nocardia asteroides*. 1. **Why Nocardia is Correct:** *Nocardia* is an aerobic, Gram-positive, filamentous bacterium that is partially acid-fast. Cotrimoxazole acts by inhibiting two consecutive steps in the bacterial synthesis of folic acid (sequential blockade). It is highly effective against *Nocardia*, and long-term therapy (6–12 months) is the standard of care for pulmonary, cutaneous, and disseminated forms of the disease. 2. **Why Actinomyces is Incorrect:** Although *Actinomyces* is also a Gram-positive filamentous bacterium, it is an **anaerobe** and is **not acid-fast**. The drug of choice for Actinomycosis is **Penicillin G** (or Amoxicillin). While Cotrimoxazole has some activity, it is not the preferred first-line agent. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Sulfamethoxazole inhibits Dihydropteroate synthase; Trimethoprim inhibits Dihydrofolate reductase. * **Other Drugs of Choice for Cotrimoxazole:** * *Pneumocystis jirovecii* (Prophylaxis and Treatment) * *Stenotrophomonas maltophilia* * *Burkholderia cepacia* * *Toxoplasma gondii* (Prophylaxis) * **Distinguishing Feature:** Remember the mnemonic **"SNAP"**: **S**ulfonamides for **N**ocardia, **A**ctinomyces treated with **P**enicillin. * **Adverse Effects:** Look out for Stevens-Johnson Syndrome (SJS), crystalluria, and megaloblastic anemia in patients with borderline folate levels.

Question 597: Gentamicin is not given orally because:

A. It has adverse effects on the gastric mucosa
B. It interferes with the absorption of other drugs
C. It is rapidly destroyed by gastric acid
D. It has poor absorption (Correct Answer)

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** Gentamicin belongs to the **Aminoglycoside** class of antibiotics. Chemically, aminoglycosides are highly **polar (poly-cationic)** molecules. Due to this high degree of ionization, they cannot cross the lipid-rich cell membranes of the gastrointestinal tract. Consequently, they have **negligible oral absorption** (less than 1%) and must be administered parenterally (IM or IV) for systemic infections. **2. Why the Incorrect Options are Wrong:** * **Option A:** Gentamicin does not cause significant direct irritation or damage to the gastric mucosa. Its primary toxicities are nephrotoxicity and ototoxicity, which occur after systemic absorption. * **Option B:** While some drugs interfere with absorption (e.g., antacids with tetracyclines), this is not the reason Gentamicin is avoided orally. In fact, oral Neomycin (another aminoglycoside) is sometimes used specifically to "sterilize" the gut because it stays within the lumen. * **Option C:** Gentamicin is actually quite stable at various pH levels and is **not destroyed by gastric acid**. The lack of efficacy via the oral route is purely a pharmacokinetic failure of absorption, not chemical degradation. **3. NEET-PG High-Yield Pearls:** * **Exception to the Rule:** Oral aminoglycosides (like Neomycin or Paromomycin) are used for **local effects** in the gut, such as hepatic encephalopathy (to kill ammonia-producing bacteria) or gut sterilization before colorectal surgery. * **Excretion:** Since they are not metabolized and are highly water-soluble, they are excreted unchanged via **glomerular filtration**. * **Spectrum:** They are primarily effective against **Aerobic Gram-negative bacilli** (they require oxygen for transport into the bacterial cell).

Question 598: Which antiretroviral drug is also effective in chronic active hepatitis B infection?

A. Zidovudine
B. Nelfinavir
C. Efavirenz
D. Lamivudine (Correct Answer)

Explanation: **Explanation:** The correct answer is **Lamivudine (3TC)**. **Why Lamivudine is correct:** Lamivudine is a Nucleoside Reverse Transcriptase Inhibitor (NRTI) primarily used in HIV treatment. However, it also possesses potent activity against the **Hepatitis B Virus (HBV)**. This dual activity occurs because the HBV polymerase enzyme functions similarly to HIV’s reverse transcriptase. Lamivudine is phosphorylated intracellularly to its active triphosphate form, which competes with deoxycytidine triphosphate for incorporation into viral DNA, leading to **chain termination** of the HBV DNA synthesis. **Why the other options are incorrect:** * **Zidovudine (AZT):** An NRTI used for HIV (especially in preventing mother-to-child transmission), but it lacks clinical efficacy against the HBV polymerase. * **Nelfinavir:** A Protease Inhibitor (PI) used in HIV. PIs target the HIV-1 protease enzyme, which is structurally distinct from any enzymes found in HBV. * **Efavirenz:** A Non-Nucleoside Reverse Transcriptase Inhibitor (NNRTI). NNRTIs bind to a specific pocket on the HIV-1 reverse transcriptase that does not exist on the HBV polymerase. **High-Yield Clinical Pearls for NEET-PG:** 1. **Dual-Action Drugs:** Other antiretrovirals effective against both HIV and HBV include **Tenofovir (TDF/TAF)** and **Emtricitabine (FTC)**. 2. **The "Flare" Phenomenon:** If a patient co-infected with HIV and HBV stops taking Lamivudine or Tenofovir, they may experience a life-threatening "flare" or exacerbation of Hepatitis B. 3. **Resistance:** Lamivudine has a low genetic barrier; long-term monotherapy for HBV often leads to the **YMDD mutation** in the HBV polymerase. Tenofovir is now preferred due to a higher barrier to resistance.

Question 599: Which antimicrobial agent's dose is reduced in renal failure?

A. Isoniazid (INH)
B. Rifabutin (Correct Answer)
C. Ethambutol
D. Kanamycin

Explanation: **Explanation** The management of antimicrobial dosing in renal failure is a high-yield topic for NEET-PG. The core principle is that drugs primarily excreted by the kidneys require dose adjustment (reduction or interval extension) to prevent accumulation and toxicity. **Why Rifabutin is the Correct Answer:** Rifabutin, a rifamycin derivative used in tuberculosis and MAC prophylaxis, undergoes significant renal excretion (approximately 10-25% as unchanged drug and metabolites). In patients with severe renal impairment (CrCl < 30 mL/min), the dose must be reduced by 50% to avoid systemic toxicity. **Analysis of Incorrect Options:** * **Isoniazid (INH):** Primarily metabolized by the liver via acetylation (NAT2 enzyme). Dose adjustment is generally not required in renal failure, though it is often supplemented after dialysis. * **Ethambutol:** While 80% is excreted renally, it is typically managed by **increasing the dosing interval** (e.g., from 24 hours to 48 hours) rather than a simple dose reduction, though some guidelines vary. However, in the context of this specific MCQ, Rifabutin is the more definitive choice for dose reduction. * **Kanamycin:** As an aminoglycoside, it is highly nephrotoxic and excreted renally. However, in clinical practice, aminoglycosides are often avoided entirely in renal failure or managed via strict Therapeutic Drug Monitoring (TDM) rather than a standardized "dose reduction" in the same manner as Rifabutin. **NEET-PG High-Yield Pearls:** 1. **Safe in Renal Failure:** Rifampicin, Isoniazid, Pyrazinamide, Doxycycline, and Ceftriaxone (dual excretion). 2. **Avoid/Adjust in Renal Failure:** Ethambutol, Aminoglycosides, Vancomycin, and Clarithromycin. 3. **Rifabutin vs. Rifampin:** Rifabutin is preferred in HIV patients on Protease Inhibitors because it is a less potent inducer of Cytochrome P450 enzymes compared to Rifampin.

Question 600: Which of the following adverse reactions absolutely contraindicates further use of rifampicin in the treatment of tuberculosis?

A. Respiratory syndrome (Correct Answer)
B. Cutaneous syndrome
C. Flu-like syndrome
D. Abdominal syndrome

Explanation: **Explanation:** Rifampicin is a cornerstone of anti-tubercular therapy (ATT), but it is associated with several immune-mediated adverse reactions, particularly when administered intermittently or after a period of drug interruption. These reactions are categorized into four major syndromes: 1. **Respiratory Syndrome (Correct Answer):** This is the most severe reaction and is characterized by dyspnea, wheezing, and occasionally pulmonary edema or shock. It is considered a **life-threatening hypersensitivity reaction**. Because of the high risk of fatal anaphylaxis or severe respiratory failure upon re-exposure, it is an **absolute contraindication** to further use of the drug. 2. **Cutaneous Syndrome (Option B):** Presents as flushing, itching, or a rash. While uncomfortable, it is usually self-limiting and can often be managed with antihistamines without discontinuing the drug. 3. **Flu-like Syndrome (Option C):** Characterized by fever, chills, and body aches. It is the most common reaction to intermittent dosing. It is generally managed by switching to a daily dosing schedule rather than permanent discontinuation. 4. **Abdominal Syndrome (Option D):** Involves nausea, vomiting, and abdominal pain. It is usually mild and does not necessitate stopping the drug unless it is associated with hepatotoxicity. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Inhibits DNA-dependent RNA polymerase. * **Other Absolute Contraindications:** Acute Renal Failure and Hemolytic Anemia (both are rare but serious immune-mediated reactions). * **Red-Orange Discoloration:** Rifampicin causes harmless orange-red discoloration of urine, sweat, and tears; patients should be warned to avoid staining contact lenses. * **Enzyme Induction:** It is a potent inducer of Cytochrome P450 enzymes, leading to significant drug interactions (e.g., reducing the efficacy of oral contraceptives and warfarin).

Practice by Chapter

Beta-Lactam Antibiotics

Practice Questions

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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