Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 1261: Levamisole is used for all except which of the following?

A. Single dose in ascariasis
B. Immunostimulant
C. Paralysis of worms by depolarization
D. Immunosuppressant in high doses (Correct Answer)

Explanation: **Explanation:** Levamisole is a synthetic imidazothiazole derivative primarily used as an anthelmintic and an immunomodulator. **Why Option D is correct:** Levamisole acts as an **immunostimulant**, not an immunosuppressant. It restores depressed T-cell function and enhances phagocytosis. While it was historically used in conditions like rheumatoid arthritis or as an adjuvant in colon cancer, it does not suppress the immune system even at high doses. In fact, high doses or prolonged use are associated with serious adverse effects like **agranulocytosis** and **levamisole-induced vasculopathy**. **Why the other options are incorrect:** * **Option A:** Levamisole is highly effective against *Ascaris lumbricoides*. A **single dose** (150 mg for adults) achieves a high cure rate (approx. 90%) by causing rapid expulsion of the worms. * **Option B:** It is a well-known **immunomodulator** that "normalizes" the immune response rather than suppressing it. It was the first drug used to boost host defense in malignancies. * **Option C:** Its mechanism of action involves acting as a **nicotinic acetylcholine receptor agonist**. This causes persistent depolarization, leading to **spastic paralysis** of the worm, which is then expelled by peristalsis. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** Levamisole is a secondary drug for Ascariasis (Albendazole/Mebendazole are preferred). * **Side Effect:** Agranulocytosis is a rare but classic board-exam side effect. * **Veterinary Use:** It is frequently used as a dewormer in livestock. * **Adulterant:** It is commonly used as a cutting agent in illicit **cocaine**, leading to characteristic retiform purpura and skin necrosis in users.

Question 1262: A 26-year-old patient presents with suspected pneumococcal meningitis. CSF culture is sent for antibiotic sensitivity. Which empirical antibiotic should be given until culture sensitivity results are available?

A. Penicillin G
B. Ceftriaxone plus metronidazole
C. Doxycycline
D. Cefotaxime plus vancomycin (Correct Answer)

Explanation: **Explanation:** The management of bacterial meningitis is a medical emergency requiring immediate empirical therapy. The choice of antibiotics is guided by the most likely pathogens and current resistance patterns. **Why D is correct:** In a 26-year-old (adult), the most common cause of bacterial meningitis is *Streptococcus pneumoniae*. Due to the rising prevalence of **Penicillin-resistant Streptococcus pneumoniae (PRSP)** and Cephalosporin-resistant strains, monotherapy is no longer recommended. * **Cefotaxime (or Ceftriaxone):** A 3rd generation cephalosporin that provides excellent CSF penetration and coverage against most pneumococci and *Neisseria meningitidis*. * **Vancomycin:** Added to the empirical regimen to cover highly penicillin-resistant pneumococci. This combination ensures synergistic and comprehensive coverage until sensitivities are known. **Why other options are incorrect:** * **A. Penicillin G:** Once the drug of choice, it is no longer used empirically due to widespread high-level resistance in *S. pneumoniae*. * **B. Ceftriaxone plus metronidazole:** Metronidazole is used for anaerobic coverage (e.g., brain abscess), but it is not part of the standard empirical protocol for community-acquired meningitis. * **C. Doxycycline:** This is a bacteriostatic drug with poor CSF penetration; it is not indicated for acute bacterial meningitis. **High-Yield Clinical Pearls for NEET-PG:** 1. **Steroids:** Dexamethasone should be administered **before or with the first dose** of antibiotics to reduce neurological sequelae (hearing loss/edema) caused by inflammatory lysis of bacteria. 2. **Age-specific coverage:** If the patient were >50 years or immunocompromised, **Ampicillin** must be added to cover *Listeria monocytogenes*. 3. **Prophylaxis:** Rifampicin is the drug of choice for chemoprophylaxis in close contacts of *Meningococcal* meningitis.

Question 1263: Which of the following anti-tubercular drugs is avoided in an HIV-positive patient on zidovudine, lamivudine, and indinavir therapy who develops TB?

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

Explanation: The correct answer is **Rifampicin**. The core medical concept here is **drug-drug interactions** mediated by the Cytochrome P450 (CYP450) enzyme system. **Why Rifampicin is avoided:** Rifampicin is a **potent inducer of CYP3A4** and other microsomal enzymes. Indinavir is a Protease Inhibitor (PI) that is metabolized by CYP3A4. When co-administered, Rifampicin significantly decreases the plasma concentration of Indinavir (by up to 90%), leading to sub-therapeutic levels, risk of treatment failure, and development of HIV drug resistance [1]. In clinical practice, **Rifabutin** is preferred over Rifampicin in patients on PIs because it is a much weaker enzyme inducer [1]. **Why other options are incorrect:** * **Pyrazinamide (A), Isoniazid (B), and Ethambutol (C):** These are standard first-line anti-tubercular drugs (ATT) that do not significantly induce or inhibit the metabolism of Protease Inhibitors or Nucleoside Reverse Transcriptase Inhibitors (NRTIs) like Zidovudine and Lamivudine [1]. They can be safely used in this regimen. **High-Yield Clinical Pearls for NEET-PG:** * **Rifampicin & PIs:** Never combine Rifampicin with Protease Inhibitors (except ritonavir-boosted lopinavir in specific doses, though Rifabutin remains the drug of choice). * **NRTIs:** Drugs like Zidovudine and Lamivudine do not undergo CYP metabolism; therefore, Rifampicin does not affect their levels. However, Zidovudine and Isoniazid both cause anemia/sideroblastic changes, requiring monitoring. * **Rifabutin:** The preferred substitute for Rifampicin in HIV patients on PI-based or NNRTI-based HAART [1]. * **Efavirenz:** If a patient is on an NNRTI-based regimen, Efavirenz is the preferred agent to use alongside Rifampicin (dose adjustment may be needed).

Question 1264: Pentamidine therapy causes all the following findings, EXCEPT?

A. Upper lobe infiltrates
B. Cavitation
C. Extrathoracic manifestations
D. None of the above (Correct Answer)

Explanation: **Explanation:** Pentamidine is a second-line agent used for the treatment and prophylaxis of *Pneumocystis jirovecii* pneumonia (PCP), particularly in patients who cannot tolerate Trimethoprim-Sulfamethoxazole (TMP-SMX). The correct answer is **"None of the above"** because Pentamidine therapy, especially when administered via the **aerosolized (inhaled) route**, is paradoxically associated with all the findings listed in options A, B, and C. 1. **Upper lobe infiltrates (Option A):** When Pentamidine is inhaled, the drug distribution is often poor in the lung apices due to gravity and ventilation-perfusion mismatch. Consequently, prophylaxis fails in the upper lobes, leading to localized PCP recurrences presenting as upper lobe infiltrates (mimicking tuberculosis). 2. **Cavitation (Option B):** Pentamidine-treated patients often show atypical radiological presentations. The inflammatory response and tissue necrosis associated with breakthrough infections can lead to thin-walled cysts (pneumatoceles) or cavitary lesions, increasing the risk of spontaneous pneumothorax. 3. **Extrathoracic manifestations (Option C):** Aerosolized Pentamidine provides only local protection [1]. It does not reach systemic circulation in therapeutic levels, allowing *P. jirovecii* to disseminate to extrapulmonary sites such as the spleen, liver, bone marrow, and lymph nodes. **High-Yield Clinical Pearls for NEET-PG:** * **Systemic Side Effects (IV Pentamidine):** Nephrotoxicity, severe hypotension (if infused rapidly), and pancreatic toxicity (initial hypoglycemia followed by secondary Diabetes Mellitus due to islet cell damage). * **Aerosolized Side Effects:** Bronchospasm (can be pre-treated with β2-agonists) [1]. * **Drug of Choice for PCP:** TMP-SMX remains the gold standard for both treatment and prophylaxis. Pentamidine is reserved for sulfa-allergic patients or treatment failure.

Question 1265: Folic acid metabolism is inhibited by which of the following agents?

A. Sulfonamides, Trimethoprim, and Nitrous oxide
B. Sulfonamides, Methotrexate, and Trimethoprim (Correct Answer)
C. Methotrexate, Nitrous oxide, and 5-Flucytosine
D. Sulfonamides, 5-Flucytosine, and Nitrous oxide

Explanation: **Explanation:** The synthesis and utilization of folic acid are critical for DNA synthesis (purine and pyrimidine production). This pathway is a major target for several antimicrobial and antineoplastic agents. 1. **Why Option B is Correct:** * **Sulfonamides:** These are structural analogs of PABA (Para-aminobenzoic acid). They competitively inhibit the enzyme **Dihydropteroate synthase**, preventing the synthesis of dihydrofolic acid. * **Trimethoprim:** This agent inhibits the enzyme **Dihydrofolate reductase (DHFR)**, preventing the conversion of dihydrofolate to the active tetrahydrofolate (THF). * **Methotrexate:** A potent folate antimetabolite used in chemotherapy and autoimmune diseases; it also inhibits **Dihydrofolate reductase** in human cells (and to some extent in microbes). Together, these drugs represent a sequential blockade of the folic acid pathway. 2. **Why Other Options are Incorrect:** * **Nitrous oxide (Options A, C, D):** While it affects Vitamin B12 (by oxidizing the cobalt atom), it does not directly inhibit the folic acid metabolic enzymes. It leads to a "folate trap" secondary to B12 inactivation, but is not classified as a primary folate metabolism inhibitor in this context. * **5-Flucytosine (Options C, D):** This is an antifungal agent that is converted to 5-fluorouracil, which inhibits **Thymidylate synthase**. While related to DNA synthesis, it is not considered a direct inhibitor of the folic acid synthesis pathway itself. **High-Yield NEET-PG Pearls:** * **Sequential Blockade:** The combination of Sulfamethoxazole and Trimethoprim (Cotrimoxazole) provides a synergistic effect by blocking two successive steps in the same metabolic pathway. * **Pyrimethamine:** Another DHFR inhibitor, primarily used for Toxoplasmosis and Malaria. * **Leucovorin (Folinic acid) Rescue:** Used to bypass the DHFR inhibition caused by high-dose Methotrexate in human cells, as it provides a pre-reduced source of folate.

Question 1266: What is the mechanism of action of ciprofloxacin in Gram-negative bacteria?

A. Inhibition of mycolic acid synthesis
B. Inhibition of topoisomerase IV
C. Inhibition of DNA gyrase (Correct Answer)
D. Inhibition of helicase

Explanation: **Ciprofloxacin** belongs to the Fluoroquinolone class of antibiotics. Its primary mechanism involves the inhibition of bacterial DNA synthesis [3].1. **Why Option C is Correct:**In **Gram-negative bacteria**, the primary target of ciprofloxacin is **DNA gyrase (Topoisomerase II)** [1, 2, 3]. DNA gyrase is responsible for introducing negative supercoils into the DNA molecule, which helps relieve the torsional strain (positive supercoiling) that develops ahead of the replicating fork [1, 2]. By inhibiting the A-subunit of DNA gyrase, ciprofloxacin prevents DNA replication and transcription, leading to bacterial cell death (bactericidal action) [1].2. **Why Other Options are Incorrect:** * **Option A:** Inhibition of mycolic acid synthesis is the mechanism of **Isoniazid (INH)**, used in the treatment of tuberculosis. * **Option B:** While fluoroquinolones also inhibit **Topoisomerase IV**, this is the *primary* target in **Gram-positive bacteria** (e.g., *S. aureus*). In Gram-negative organisms like *E. coli*, DNA gyrase is the more sensitive target [1]. * **Option D:** Helicase is responsible for unwinding the DNA double helix, but it is not the target of fluoroquinolones.**High-Yield Clinical Pearls for NEET-PG:** * **Spectrum:** Ciprofloxacin is highly active against aerobic Gram-negative bacilli, including *Pseudomonas aeruginosa*. * **Resistance:** Occurs via mutations in the *gyrA* gene or through "Qnr proteins" (plasmid-mediated). * **Adverse Effects:** Most characteristic are **tendon rupture/tendonitis** (especially in elderly or those on steroids) and **QT interval prolongation**. * **Contraindication:** Generally avoided in children and pregnancy due to the risk of **arthropathy** (cartilage damage).

Question 1267: What is the drug of choice for the treatment of meningococcal meningitis?

A. Piperacillin
B. Ceftriaxone (Correct Answer)
C. Ampicillin
D. Meropenem

Explanation: **Explanation:** **Ceftriaxone (Option B)** is the drug of choice for meningococcal meningitis caused by *Neisseria meningitidis*. The underlying medical concept relies on its excellent **cerebrospinal fluid (CSF) penetration**, long half-life (allowing once or twice daily dosing), and high potency against common meningeal pathogens. While penicillin G was historically the gold standard, the rise of penicillin-resistant strains has made third-generation cephalosporins like Ceftriaxone the empirical and definitive treatment of choice. **Analysis of Incorrect Options:** * **Piperacillin (Option A):** This is an antipseudonal penicillin. While it has a broad spectrum, it is primarily used for *Pseudomonas aeruginosa* infections and does not achieve optimal therapeutic levels in the CSF compared to Ceftriaxone. * **Ampicillin (Option C):** This is the drug of choice for *Listeria monocytogenes* meningitis (common in neonates and the elderly) but is not the primary choice for meningococcus due to potential resistance. * **Meropenem (Option D):** While it has excellent CSF penetration and covers *N. meningitidis*, it is reserved for multi-drug resistant (MDR) cases or patients with severe beta-lactam allergies (cephalosporin resistance). It is not the first-line agent. **High-Yield Clinical Pearls for NEET-PG:** * **Chemoprophylaxis:** For close contacts of a patient with meningococcal meningitis, **Rifampicin** is the drug of choice (alternatives: Ciprofloxacin or Ceftriaxone). * **Empirical Therapy:** In adults, the standard empirical regimen is **Ceftriaxone + Vancomycin** (to cover resistant *S. pneumoniae*). * **Steroids:** Dexamethasone should be administered just before or with the first dose of antibiotics to reduce neurological complications (primarily in *H. influenzae* and *S. pneumoniae*).

Question 1268: Which of the following most often causes cholestatic jaundice?

A. Isoniazid (INH)
B. Erythromycin estolate (Correct Answer)
C. Pyrazinamide
D. Ethionamide

Explanation: **Explanation:** The correct answer is **Erythromycin estolate**. **1. Why Erythromycin estolate is correct:** Cholestatic jaundice is a classic, well-documented adverse effect specifically associated with the **estolate salt** of Erythromycin. It is considered a hypersensitivity reaction characterized by fever, abdominal pain, and jaundice, usually occurring after 1–3 weeks of therapy. The estolate form is more prone to causing this than other salts (like stearate or succinate) because it acts as a mild hepatotoxin and a sensitizing agent, leading to bile stasis in the canaliculi. **2. Why the other options are incorrect:** * **A. Isoniazid (INH):** While INH is notoriously hepatotoxic, it typically causes **hepatocellular necrosis** (elevated ALT/AST) rather than cholestasis. The risk increases with age and alcohol consumption. * **C. Pyrazinamide:** This is the most hepatotoxic of the first-line anti-tubercular drugs (ATT). Like INH, it causes **dose-dependent hepatotoxicity** and liver necrosis, not specifically a cholestatic pattern. * **D. Ethionamide:** A second-line ATT that can cause hepatitis in about 5% of patients, but it is not the "most common" or classic cause of cholestatic jaundice among the choices provided. **3. NEET-PG High-Yield Pearls:** * **Macrolide of choice in pregnancy:** Azithromycin (Erythromycin estolate is contraindicated in pregnancy due to the risk of cholestasis). * **Motilin Agonist:** Erythromycin acts on motilin receptors, causing prokinetic effects (diarrhea/cramps). * **Enzyme Inhibition:** Erythromycin is a potent **CYP3A4 inhibitor**, leading to numerous drug interactions (e.g., increasing levels of Theophylline, Warfarin, and Statins). * **Other drugs causing Cholestatic Jaundice:** Chlorpromazine, Methyltestosterone, and Oral Contraceptive Pills (OCPs).

Question 1269: FK 506 is a type of:

A. Immunoglobulin antibody
B. Non-depolarizing muscle relaxant
C. Macrolide antibiotic (Correct Answer)
D. Opioid anaesthetic

Explanation: **Explanation:** **FK 506**, also known as **Tacrolimus**, is a potent immunosuppressant derived from the fungus *Streptomyces tsukubaensis*. Chemically, it is classified as a **macrolide antibiotic**, though it lacks significant antibacterial activity. Its primary clinical utility lies in its ability to prevent organ transplant rejection (especially liver and kidney). **Mechanism of Action:** Tacrolimus acts as a **calcineurin inhibitor**. It binds to an intracellular protein called **FK-binding protein (FKBP-12)**. This complex inhibits calcineurin, a phosphatase required for the activation of the transcription factor NFAT (Nuclear Factor of Activated T-cells). This prevents the transcription of **Interleukin-2 (IL-2)** and other cytokines, thereby inhibiting T-lymphocyte activation. **Analysis of Incorrect Options:** * **Option A (Immunoglobulin antibody):** Tacrolimus is a small-molecule drug, not a protein-based antibody (like Basiliximab or Muromonab-CD3). * **Option B (Non-depolarizing muscle relaxant):** These agents (e.g., Vecuronium, Atracurium) act on nicotinic receptors at the neuromuscular junction; Tacrolimus has no such activity. * **Option D (Opioid anaesthetic):** Opioids (e.g., Fentanyl) act on mu, kappa, and delta receptors for analgesia; Tacrolimus is not used for pain or anesthesia. **High-Yield NEET-PG Pearls:** * **Potency:** Tacrolimus is 10–100 times more potent than Cyclosporine. * **Side Effects:** Nephrotoxicity (most common), neurotoxicity (tremors, seizures), and **new-onset diabetes after transplantation (NODAT)**. Unlike Cyclosporine, it does *not* typically cause hirsutism or gum hyperplasia. * **Topical Use:** Used in ointment form for **Atopic Dermatitis**.

Question 1270: Which antimicrobial agent inhibits ergosterol biosynthesis?

A. Ketoconazole (Correct Answer)
B. Amphotericin B
C. 5-Flucytosine
D. Griseofulvin

Explanation: ### Explanation **Correct Option: A. Ketoconazole** Ketoconazole is an **Azole** antifungal. The mechanism of action for all azoles involves the inhibition of the fungal cytochrome P450 enzyme, **14-α-demethylase**. This enzyme is responsible for converting lanosterol into **ergosterol** (a vital component of the fungal cell membrane). By blocking this pathway, azoles deplete ergosterol, leading to increased membrane permeability and fungal cell death. **Why the other options are incorrect:** * **B. Amphotericin B:** This is a Polyene antibiotic. It does not inhibit the *synthesis* of ergosterol; instead, it **binds directly to pre-formed ergosterol** in the fungal cell membrane, creating pores that cause leakage of intracellular ions (like $K^+$). * **C. 5-Flucytosine:** This is an antimetabolite. It is converted into 5-fluorouracil (5-FU) inside the fungal cell, which **inhibits DNA and RNA synthesis** by interfering with thymidylate synthase. * **D. Griseofulvin:** This agent acts by **interfering with microtubule function**, thereby inhibiting mitosis (spindle formation) in fungi. It is primarily used for dermatophytosis. **High-Yield Clinical Pearls for NEET-PG:** * **Ketoconazole Side Effects:** It is a potent inhibitor of human CYP450 enzymes and steroidogenesis, leading to **gynecomastia**, decreased libido, and menstrual irregularities. * **Drug of Choice (DOC):** While Ketoconazole was the first oral azole, **Itraconazole** is now the DOC for most systemic mycoses (like Histoplasmosis), and **Fluconazole** is the DOC for Cryptococcal meningitis and Candidiasis. * **Terbinafine:** Another ergosterol synthesis inhibitor, but it acts earlier in the pathway by inhibiting **Squalene epoxidase**.

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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