Antimicrobial Agents — MCQs
On this page
What is the mechanism of action of Dabigatran?
Anterior nares is the most common site of colonization in the carrier state of MRSA. What is the best treatment?
Which of the following anti-HIV drugs can lead to nephrolithiasis?
Which of the following is the drug of choice for the treatment of corneal ulcers caused by filamentous fungi?
Penicillins inhibit which of the following bacterial processes/compounds?
All of the following drugs are contraindicated in patients with G-6-PD deficiency, except?
What is the mechanism of Acyclovir resistance?
Which of the following cephalosporins can cause thrombocytopenia?
Erythromycin acts on which receptor in the GITract?
If a drug is active against the pre-erythrocytic stage of the malaria parasite, it will be useful as:
Antimicrobial Agents Indian Medical PG Practice Questions and MCQs
Question 621: What is the mechanism of action of Dabigatran?
- A. Activation of plasmin
- B. Direct inhibition of thrombin (Correct Answer)
- C. Indirect inhibition of thrombin
- D. Inactivation of vitamin K dependent factors
Explanation: **Explanation:** **Correct Answer: B. Direct inhibition of thrombin** Dabigatran etexilate is an oral, reversible, **Direct Thrombin Inhibitor (DTI)** [1], [2]. Unlike heparin, which requires antithrombin III as a cofactor, Dabigatran binds directly to the active site of both free and clot-bound thrombin (Factor IIa) [1], [2]. By inhibiting thrombin, it prevents the conversion of fibrinogen to fibrin, thereby blocking the final step of the coagulation cascade. **Analysis of Incorrect Options:** * **A. Activation of plasmin:** This is the mechanism of **Thrombolytics** (e.g., Streptokinase, Alteplase), which convert plasminogen to plasmin to dissolve existing clots. * **C. Indirect inhibition of thrombin:** This describes **Unfractionated Heparin (UFH)** and **Low Molecular Weight Heparins (LMWH)**. They act indirectly by accelerating the activity of Antithrombin III [3]. * **D. Inactivation of vitamin K dependent factors:** This is the mechanism of **Warfarin**, which inhibits Vitamin K Epoxide Reductase (VKOR), preventing the synthesis of Factors II, VII, IX, and X. **High-Yield NEET-PG Pearls:** * **Prodrug Status:** Dabigatran etexilate is a prodrug converted by esterases to its active form, Dabigatran [1], [2]. * **Monitoring:** Routine monitoring (like PT/INR) is not required, but **aPTT** can be used to assess its effect in emergencies [2]. * **Specific Antidote:** **Idarucizumab** (a humanized monoclonal antibody fragment) is the specific reversal agent for Dabigatran. * **Excretion:** It is primarily eliminated by the **kidneys**; thus, it is contraindicated in severe renal impairment [1]. * **Clinical Use:** Primarily used for stroke prevention in non-valvular atrial fibrillation and treatment of DVT/PE [1].
Question 622: Anterior nares is the most common site of colonization in the carrier state of MRSA. What is the best treatment?
- A. Topical mupirocin (Correct Answer)
- B. Oral Vancomycin
- C. Inhaled colistin
- D. IV cefazolin
Explanation: The anterior nares serve as the primary reservoir for *Staphylococcus aureus*, including Methicillin-resistant *S. aureus* (MRSA) [1]. To eradicate the carrier state and prevent the spread of infection (especially in healthcare workers or pre-operative patients), a localized, high-concentration treatment is required. **Mupirocin (Pseudomonic acid A)** is the drug of choice for this purpose [2]. It works by inhibiting **isoleucyl-tRNA synthetase**, thereby halting bacterial protein synthesis. Its topical application ensures high local concentrations that effectively clear the colonization without systemic side effects. **2. Why the Other Options are Incorrect:** * **Oral Vancomycin:** Vancomycin is not absorbed from the GI tract [1]. It is used orally only for *Clostridioides difficile* colitis. For systemic MRSA, it must be given IV [1]. It is not used for nasal decolonization. * **Inhaled Colistin:** This is used for treating multi-drug resistant Gram-negative infections (like *Pseudomonas*) in cystic fibrosis or ventilator-associated pneumonia, not for Gram-positive nasal colonization [2]. * **IV Cefazolin:** Cefazolin is a first-generation cephalosporin. By definition, **MRSA is resistant to all beta-lactams** (except 5th generation cephalosporins like Ceftaroline) due to the altered penicillin-binding protein (PBP2a).
Question 623: Which of the following anti-HIV drugs can lead to nephrolithiasis?
- A. Raltegravir
- B. Indinavir (Correct Answer)
- C. Raltegravir
- D. Enfuvirtide
Explanation: **Explanation:** **Indinavir**, a Protease Inhibitor (PI), is the correct answer. The primary reason for its association with nephrolithiasis (kidney stones) is its **poor solubility** in urine, especially at physiological pH. Approximately 20% of patients taking Indinavir develop crystalluria or radiolucent renal stones composed of the drug itself. To mitigate this risk, patients are strictly advised to maintain high fluid intake (at least 1.5–2 liters of water daily). **Analysis of Incorrect Options:** * **Raltegravir (Options A & C):** This is an Integrase Strand Transfer Inhibitor (INSTI). Its primary side effects include nausea, headache, and a potential increase in creatine kinase. It is not associated with renal stone formation. * **Enfuvirtide (Option D):** This is a Fusion Inhibitor administered subcutaneously. Its most common side effects are local injection site reactions (nodules, erythema) and an increased risk of bacterial pneumonia, but it does not cause nephrolithiasis. **NEET-PG High-Yield Pearls:** * **Indinavir Side Effects:** Beyond nephrolithiasis, it is known for causing **hyperbilirubinemia** (indirect) and "Buffalo Hump" (lipodystrophy), a class effect of PIs. * **Protease Inhibitor Class Effects:** Most PIs (except Atazanavir) are associated with metabolic syndrome, including hyperglycemia, hyperlipidemia, and insulin resistance. * **Atazanavir:** Another PI that can cause nephrolithiasis and hyperbilirubinemia, but it is preferred over Indinavir due to better dosing and fewer side effects. * **Tenofovir (NRTI):** While not an option here, remember it causes **Fanconi Syndrome** (proximal renal tubule dysfunction), which is a frequent renal-related question in NEET-PG.
Question 624: Which of the following is the drug of choice for the treatment of corneal ulcers caused by filamentous fungi?
- A. Itraconazole
- B. Natamycin (Correct Answer)
- C. Nystatin
- D. Ketoconazole
Explanation: Natamycin is the drug of choice for fungal keratitis (corneal ulcers) caused by filamentous fungi, such as Aspergillus and Fusarium species. It is a polyene antifungal that works by binding to ergosterol in the fungal cell membrane, leading to cell lysis. It is preferred because it is the only topical antifungal formulation specifically FDA-approved for ophthalmic use and demonstrates superior efficacy in clinical trials (notably the MUTT trial) compared to other agents for filamentous infections. Analysis of Incorrect Options: * Itraconazole (A): While effective against many fungi, it has poor ocular penetration when given topically. It is generally reserved as an oral adjunct for deep-seated or severe infections. * Nystatin (C): Although a polyene like natamycin, it is primarily used for mucocutaneous candidiasis (yeast). It is not the standard of care for filamentous corneal ulcers due to its spectrum and lack of suitable ophthalmic preparations. * Ketoconazole (D): This is an older azole with significant side effects and lower efficacy compared to newer triazoles or natamycin. It is rarely used as a primary treatment for fungal keratitis. High-Yield Clinical Pearls for NEET-PG: * Drug of Choice for Yeast (Candida) Keratitis: Topical Amphotericin B (0.15%) or Fluconazole [2, 5]. * Gold Standard Diagnosis: Corneal scraping followed by KOH mount and culture on Sabouraud’s Dextrose Agar (SDA). * Clinical Sign: Fungal ulcers often present with feathery margins, satellite lesions, and an immune ring (Wessely ring). * Contraindication: Never use topical steroids in suspected fungal keratitis, as they worsen the infection and can lead to corneal perforation.
Question 625: Penicillins inhibit which of the following bacterial processes/compounds?
- A. Topoisomerase
- B. Protein synthesis
- C. Cell-wall synthesis (Correct Answer)
- D. Dihydropteroate synthase
Explanation: **Explanation:** **Mechanism of Action (Why C is correct):** Penicillins belong to the **Beta-lactam** class of antibiotics. Their primary mechanism of action is the inhibition of bacterial **cell-wall synthesis**. They act as structural analogs of the D-alanyl-D-alanine sequence. Penicillins bind to and inhibit **Penicillin-Binding Proteins (PBPs)**, specifically the **transpeptidase** enzyme. This prevents the cross-linking of peptidoglycan chains, which is essential for maintaining the structural integrity of the bacterial cell wall. This leads to osmotic lysis and bacterial death (**Bactericidal** action). **Analysis of Incorrect Options:** * **A. Topoisomerase:** This is the target of **Fluoroquinolones** (e.g., Ciprofloxacin). They inhibit DNA Gyrase (Topoisomerase II) and Topoisomerase IV, interfering with DNA replication. * **B. Protein synthesis:** This is the target of several classes, including **Aminoglycosides, Tetracyclines, Macrolides, and Chloramphenicol**, which act on the 30S or 50S ribosomal subunits. * **D. Dihydropteroate synthase:** This enzyme is inhibited by **Sulfonamides**. It is a crucial step in the bacterial folic acid synthesis pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Resistance Mechanism:** The most common cause of resistance to penicillins is the production of **Beta-lactamases** (penicillinases) by bacteria. * **Synergy:** Penicillins are often combined with **Aminoglycosides** (e.g., in Enterococcal endocarditis) because cell-wall inhibition facilitates the entry of aminoglycosides into the cell. * **Probenecid Interaction:** Probenecid decreases the renal tubular secretion of penicillin, thereby increasing its plasma concentration and duration of action.
Question 626: All of the following drugs are contraindicated in patients with G-6-PD deficiency, except?
- A. Cotrimoxazole
- B. Furazolidone
- C. Nalidixic acid
- D. Ceftriaxone (Correct Answer)
Explanation: The core concept behind this question is **Oxidative Stress**. Patients with Glucose-6-Phosphate Dehydrogenase (G-6-PD) deficiency cannot maintain adequate levels of reduced glutathione, which is essential for protecting Red Blood Cells (RBCs) from oxidative damage. When exposed to certain oxidizing drugs, hemoglobin denatures into **Heinz bodies**, leading to acute hemolysis [1]. **Why Ceftriaxone is the correct answer:** Ceftriaxone is a third-generation cephalosporin. Unlike sulfonamides or nitrofurans, cephalosporins do not possess significant oxidizing potential and do not interfere with the pentose phosphate pathway. Therefore, they are safe to use in G-6-PD deficient patients. **Analysis of Incorrect Options:** * **Cotrimoxazole (Sulfamethoxazole + Trimethoprim):** Sulfonamides are classic oxidizing agents and are among the most common triggers for hemolytic anemia in G-6-PD deficiency. * **Furazolidone:** This belongs to the nitrofuran class (like Nitrofurantoin). These drugs undergo redox cycling, generating free radicals that overwhelm the RBC's limited antioxidant capacity. * **Nalidixic acid:** This older quinolone is a known oxidative stressor. While newer fluoroquinolones (like Ciprofloxacin) have a lower risk, Nalidixic acid is strictly contraindicated. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for G-6-PD Triggers:** "**AAA**" – **A**ntimalarials (Primaquine, Chloroquine), **A**ntibiotics (Sulfonamides, Nitrofurantoin, Nalidixic acid), and **A**spirin (high doses) [1]. * **Other notable triggers:** Dapsone (highest risk), Rasburicase, and Fava beans (Favism). * **Peripheral Smear Findings:** Look for **"Bite cells"** (degmacytes) and **Heinz bodies** (visualized with supra-vital stains like Crystal Violet).
Question 627: What is the mechanism of Acyclovir resistance?
- A. Thymidine kinase deficiency (Correct Answer)
- B. DNA-dependent RNA polymerase mutation
- C. Altered viral DNA polymerase
- D. Drug efflux pumps
Explanation: ### Explanation **Mechanism of Action (The "Why"):** Acyclovir is a guanosine analogue that acts as a **prodrug**. To become active, it must undergo three phosphorylation steps. The first and most critical step is the conversion of Acyclovir to Acyclovir monophosphate, which is mediated by the **viral enzyme Thymidine Kinase (TK)**. Host cell enzymes then complete the conversion to the active triphosphate form, which inhibits viral DNA polymerase. The most common mechanism of resistance (especially in immunocompromised patients) is the **absence or reduced production of viral Thymidine Kinase (TK-deficient strains)**. Without this enzyme, the drug cannot be "trapped" or activated within the infected cell. **Analysis of Incorrect Options:** * **B. DNA-dependent RNA polymerase mutation:** This is the mechanism of resistance for **Rifampicin** (anti-tubercular drug), not antiviral agents acting on DNA viruses. * **C. Altered viral DNA polymerase:** While this *can* occur, it is a much less common cause of resistance than TK deficiency. It is usually seen in "TK-altered" strains where the enzyme is present but lacks affinity for the drug. * **D. Drug efflux pumps:** This is a common mechanism for **antibacterial** resistance (e.g., Tetracyclines) and cancer chemotherapy, but not a primary mechanism for Acyclovir resistance. **High-Yield Clinical Pearls for NEET-PG:** 1. **Cross-Resistance:** Strains resistant to Acyclovir due to TK deficiency are also resistant to **Valacyclovir, Famciclovir, and Ganciclovir**. 2. **Drug of Choice for Resistant Cases:** **Foscarnet** or **Cidofovir** are used because they do not require activation by viral Thymidine Kinase (they bypass the phosphorylation step). 3. **Selectivity:** Acyclovir is highly selective because it is only activated in virus-infected cells, sparing healthy host cells.
Question 628: Which of the following cephalosporins can cause thrombocytopenia?
- A. Cefazolin
- B. Cefamandole
- C. Ceftriaxone (Correct Answer)
- D. Ceftabiprole
Explanation: **Explanation:** **Ceftriaxone** is a third-generation cephalosporin widely used for its broad-spectrum activity and long half-life. While cephalosporins are generally associated with immune-mediated hemolytic anemia, **Ceftriaxone** is specifically linked to **immune-mediated thrombocytopenia**. This occurs via a drug-induced antibody mechanism where the drug (or its metabolite) binds to platelets, leading to their peripheral destruction. Additionally, Ceftriaxone is unique for causing "biliary sludge" or pseudolithiasis due to its high biliary excretion. **Analysis of Incorrect Options:** * **Cefazolin (Option A):** A first-generation cephalosporin primarily used for surgical prophylaxis. While it can rarely cause leukopenia, it is not the classic answer for drug-induced thrombocytopenia in this context. * **Cefamandole (Option B):** A second-generation cephalosporin known for its **N-methylthiotetrazole (NMTT) side chain**. This side chain causes hypoprothrombinemia (bleeding due to Vitamin K inhibition) and Disulfiram-like reactions, rather than direct thrombocytopenia. * **Ceftobiprole (Option D):** A fifth-generation cephalosporin with activity against MRSA. Its side effect profile focuses more on gastrointestinal upset and dysgeusia (taste disturbance). **High-Yield Clinical Pearls for NEET-PG:** * **NMTT Side Chain Drugs:** Cefamandole, Cefotetan, and Cefoperazone. Remember the mnemonic: *"Man, Tan, Operazone"* for bleeding risks and Disulfiram reactions. * **Ceftriaxone Contraindication:** Avoid in neonates receiving intravenous Calcium (e.g., Calcium Gluconate) due to the risk of fatal **Ceftriaxone-Calcium precipitates** in lungs and kidneys. * **Excretion:** Most cephalosporins are renally excreted; however, **Ceftriaxone and Cefoperazone** are primarily excreted in the bile and do not require dose adjustment in renal failure.
Question 629: Erythromycin acts on which receptor in the GITract?
- A. Motilin (Correct Answer)
- B. Muscarinic
- C. Nicotinic
- D. Serotonergic
Explanation: **Explanation:** **Mechanism of Action (Correct Answer: A)** Erythromycin, a macrolide antibiotic, acts as a **non-peptide motilin receptor agonist**. Motilin is a polypeptide hormone secreted by M-cells in the upper small intestine that initiates the **Migrating Motor Complex (MMC)** [4]. By binding to motilin receptors on the smooth muscle of the stomach and duodenum, Erythromycin stimulates gastrointestinal motility, effectively acting as a **prokinetic agent** [2]. **Analysis of Incorrect Options:** * **B & C (Muscarinic/Nicotinic):** These are cholinergic receptors. While drugs like Bethanechol (Muscarinic) or Neostigmine (Acetylcholinesterase inhibitor) increase GI motility via the parasympathetic system, Erythromycin does not interact with these receptors. * **D (Serotonergic):** 5-HT4 agonists (e.g., Prucalopride, Metoclopramide) and 5-HT3 antagonists (e.g., Ondansetron) are used for GI motility and emesis, respectively [1]. Erythromycin’s pathway is independent of serotonin [3]. **Clinical Pearls for NEET-PG:** * **Clinical Use:** Due to its prokinetic effect, Erythromycin is used off-label for **Diabetic Gastroparesis** and to clear the stomach of blood before endoscopy in upper GI bleeds [2]. * **Side Effect:** The most common side effect of Erythromycin is **abdominal cramps/diarrhea**, which is a direct extension of its motilin-agonist property [2]. * **Tachyphylaxis:** Long-term use as a prokinetic is limited by tachyphylaxis (rapidly diminishing response) due to down-regulation of motilin receptors. * **Drug Interaction:** It is a potent **CYP3A4 inhibitor**, leading to significant drug interactions (e.g., increasing levels of Theophylline or Warfarin) [1].
Question 630: If a drug is active against the pre-erythrocytic stage of the malaria parasite, it will be useful as:
- A. Suppressive prophylactic
- B. Causal prophylactic (Correct Answer)
- C. Clinical curative
- D. Radical curative
Explanation: ### Explanation To answer this question, it is essential to correlate the life cycle of the malaria parasite with the clinical terminology of antimalarial therapy. **1. Why "Causal Prophylactic" is correct:** The **pre-erythrocytic stage** (primary hepatic stage) occurs after a mosquito bite but *before* the parasite enters the red blood cells. Drugs that act at this stage prevent the parasite from ever reaching the bloodstream. Therefore, they prevent the initiation of the infection entirely. This is termed **Causal Prophylaxis**. * *Example:* **Primaquine** and **Atovaquone** act as causal prophylactics. **2. Why the other options are incorrect:** * **Suppressive Prophylactic:** These drugs act on the **erythrocytic (RBC) stage**. They do not prevent the liver infection but kill the parasites as they emerge into the blood, thereby "suppressing" clinical symptoms. (e.g., Chloroquine, Mefloquine). * **Clinical Curative:** These drugs are used to treat an established infection by targeting the **erythrocytic stage** to terminate a clinical attack of malaria. (e.g., Artemisinins, Quinine). * **Radical Curative:** This refers to the eradication of **hypnozoites** (latent liver stages) of *P. vivax* and *P. ovale* to prevent a relapse. While this is a liver stage, it is "exo-erythrocytic" rather than "pre-erythrocytic." (e.g., Primaquine, Tafenoquine). ### NEET-PG High-Yield Pearls: * **Primaquine** is the "all-rounder": It acts as a causal prophylactic, a radical curative, and a gametocidal agent. * **Proguanil** is a slow-acting causal prophylactic. * **Pyrimethamine** is a sporontocidal agent (prevents development in the mosquito). * **True Prophylaxis** (Causal) is rarely achieved; most travelers use **Suppressive Prophylaxis** (starting before travel and continuing for 4 weeks after returning).
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