Antimicrobial Agents — MCQs

Antimicrobial Agents Indian Medical PG Practice Questions and MCQs

Question 1581: Hypothyroidism is caused by which of the following anti-tubercular drugs?

A. Ethambutol
B. Thioacetazone
C. Streptomycin
D. Ethionamide (Correct Answer)

Explanation: ***Ethionamide*** - Ethionamide is a known **thiocarbamide derivative** that can inhibit thyroid hormone synthesis, leading to **hypothyroidism**. - It interferes with the binding of iodine to tyrosine residues within thyroglobulin, reducing the production of **T3 and T4 hormones**. *Streptomycin* - Streptomycin is an **aminoglycoside antibiotic** primarily associated with **ototoxicity** (vestibular and cochlear damage) and **nephrotoxicity**. - It does not have a well-documented association with thyroid dysfunction or hypothyroidism. *Thioacetazone* - Thioacetazone is a **second-line anti-tubercular drug** most commonly associated with severe skin reactions like **Stevens-Johnson syndrome (SJS)** and **toxic epidermal necrolysis (TEN)**. - While it can cause various side effects, hypothyroidism is not a typical or significant adverse effect. *Ethambutol* - Ethambutol's primary and most commonly recognized adverse effect is **ocular toxicity**, specifically **optic neuritis**, which can lead to red-green color blindness and decreased visual acuity. - It does not interfere with thyroid function or promote hypothyroidism.

Question 1582: A 30-year-old female patient complains of a cough with sputum, and is a known case of tuberculosis who is currently taking rifampin for the same. All of the following are true about this drug except:

A. It inhibits transcription
B. It binds with the beta-subunit of prokaryotic RNA polymerase
C. It binds with the alpha-subunit of prokaryotic RNA polymerase (Correct Answer)
D. It is metabolized by CYP450 enzymes

Explanation: ***It binds with the alpha-subunit of prokaryotic RNA polymerase*** - **Rifampin (Rifampicin)** specifically binds to the **beta-subunit** of **DNA-dependent RNA polymerase** in prokaryotes. - This binding leads to inhibition of **transcription** by preventing RNA chain elongation. *It inhibits transcription* - This statement is **true** as rifampin's primary mechanism of action is to inhibit **bacterial RNA synthesis** by blocking the transcription process. - By binding to the RNA polymerase, it prevents the synthesis of messenger RNA (mRNA). *It binds with the beta-subunit of prokaryotic RNA polymerase* - This statement is **true** and describes the precise molecular target of rifampin. - The binding to the **beta-subunit** prevents the enzyme from progressing along the DNA template. *It is metabolized by CYP450 enzymes* - This statement is **true** and is clinically significant because rifampin is a potent **inducer of various cytochrome P450 (CYP450) enzymes**. - This enzyme induction can lead to **accelerated metabolism** of many co-administered drugs, requiring dose adjustments.

Question 1583: Common dose-dependent side effects of ethambutol include which of the following?

A. Optic neuritis (Correct Answer)
B. Peripheral neuropathy
C. Red-orange urine
D. Nephrotoxicity

Explanation: ***Optic neuritis*** - **Ethambutol** is known to cause dose-dependent **optic neuritis**, leading to decreased visual acuity and **red-green color blindness**. - This side effect is typically reversible if the drug is discontinued promptly, but delayed recognition can result in permanent visual impairment. *Red-orange urine* - **Red-orange discoloration of urine** is a characteristic side effect of **rifampin**, another antituberculosis drug, not ethambutol. - This side effect is benign and not indicative of organ dysfunction. *Nephrotoxicity* - **Nephrotoxicity** (kidney damage) is not a common or characteristic side effect of **ethambutol**. - Medications like **aminoglycosides** (e.g., gentamicin) and **NSAIDs** are more commonly associated with nephrotoxic effects. *Peripheral neuropathy* - **Peripheral neuropathy** is a common side effect associated with **isoniazid**, another antituberculosis medication. - It is often prevented or treated with concurrent administration of **pyridoxine (Vitamin B6)**.

Question 1584: Which of the following drugs acts against Pseudomonas?

A. Piperacillin (Correct Answer)
B. Cloxacillin
C. Methicillin
D. Nafcillin

Explanation: ***Piperacillin*** - **Piperacillin** is an extended-spectrum penicillin often combined with **tazobactam** to protect against beta-lactamase degradation. - It has significant activity against **Pseudomonas aeruginosa**, making it a key antibiotic for severe **Gram-negative infections**. *Methicillin* - **Methicillin** is a narrow-spectrum penicillinase-resistant penicillin primarily used for **MSSA (methicillin-sensitive Staphylococcus aureus)** infections. - It lacks activity against **Pseudomonas aeruginosa** and is associated with significant **nephrotoxicity**. *Cloxacillin* - **Cloxacillin** is a narrow-spectrum, **beta-lactamase-resistant penicillin** used for **Staphylococcal infections**, similar to methicillin. - It is **ineffective** against **Pseudomonas aeruginosa**, which has a different cell wall structure and resistance mechanisms. *Nafcillin* - **Nafcillin** is another **penicillinase-resistant penicillin** used mainly for serious **Staphylococcal infections** like endocarditis. - It does not offer coverage against **Pseudomonas aeruginosa** and is often administered intravenously for severe infections.

Question 1585: Which of the following drugs does not act on microtubules?

A. Colchicine
B. Fluconazole (Correct Answer)
C. Paclitaxel
D. Vinblastine

Explanation: ***Fluconazole*** - **Fluconazole** is an **antifungal drug** [2] that acts by inhibiting **14-alpha-demethylase**, an enzyme crucial for **ergosterol synthesis** [1] in fungal cell membranes. - It does not interact with **microtubules** in any way; its mechanism is distinct from agents affecting cellular structural components. *Colchicine* - **Colchicine** is an anti-inflammatory drug primarily used for **gout**, which works by binding to **tubulin** and inhibiting its polymerization into **microtubules**. - This action disrupts **neutrophil migration**, phagocytosis, and degranulation, thereby reducing inflammation. *Vinblastine* - **Vinblastine** is a **vinca alkaloid** chemotherapy agent that inhibits **microtubule formation** by preventing the polymerization of tubulin. - This disruption leads to the arrest of cells in **metaphase** during mitosis, making it an effective anticancer drug. *Paclitaxel* - **Paclitaxel** is a **taxane** chemotherapy agent that stabilizes **microtubules** by preventing their depolymerization. - This stabilization arrests cells in the **G2/M phase** (particularly metaphase), inhibiting **cell division** and leading to apoptotic cell death.

Question 1586: Carbapenem with the maximum seizure risk is:

A. Imipenem (Correct Answer)
B. Meropenem
C. Doripenem
D. Ertapenem

Explanation: ***Imipenem*** - **Imipenem** is primarily associated with a higher risk of seizures due to its ability to inhibit **GABAergic neurotransmission** in the central nervous system. - This effect is dose-dependent and more pronounced in patients with **renal impairment** or pre-existing CNS disorders, where drug accumulation occurs. *Meropenem* - While meropenem can also cause seizures, its **GABA antagonistic effect** is less potent than imipenem, resulting in a lower incidence of this adverse event. - It is generally considered safer than imipenem for patients at risk of seizures. *Ertapenem* - Ertapenem has an even lower propensity for causing seizures compared to imipenem and meropenem. - It is often preferred in outpatient settings due to its **once-daily dosing** and favorable safety profile regarding CNS adverse effects. *Doripenem* - Doripenem also has a **low seizure potential**, similar to meropenem and ertapenem. - Its **pharmacokinetic profile** and CNS penetration differ, but it is not associated with the same high risk as imipenem.

Question 1587: Bacitracin acts on:

A. Cell wall (Correct Answer)
B. Cell membrane
C. Nucleic acid
D. Ribosome

Explanation: ***Cell wall*** - **Bacitracin** inhibits bacterial **cell wall synthesis** by interfering with the dephosphorylation of the C55-isoprenyl pyrophosphate lipid carrier molecule. - This lipid carrier is essential for transporting **peptidoglycan** precursors across the bacterial membrane to the growing cell wall. *Cell membrane* - While bacitracin does target the synthesis pathway for the cell wall which is adjacent to the membrane, its direct action is not on the integrity or function of the **cell membrane** itself. - Drugs that act directly on the cell membrane typically disrupt its permeability, leading to leakage of cytoplasmic contents. *Nucleic acid* - **Nucleic acid** synthesis inhibitors (e.g., fluoroquinolones, rifampin) target DNA replication, transcription, or repair mechanisms. - Bacitracin does not interfere with the synthesis or function of bacterial **DNA** or **RNA**. *Ribosome* - **Ribosome** inhibitors (e.g., macrolides, tetracyclines) target bacterial protein synthesis, binding to either the 30S or 50S ribosomal subunit. - Bacitracin's mechanism of action is distinct from protein synthesis inhibition.

Question 1588: Ketoconazole is a medication used for which of the following purposes?

A. Antifungal agent (Correct Answer)
B. Antibacterial
C. Antiviral agent
D. Chemotherapeutic agent

Explanation: ***Antifungal agent*** - **Ketoconazole** is a broad-spectrum imidazole antifungal agent used to treat various **superficial and systemic fungal infections**. - It works by inhibiting the synthesis of **ergosterol**, a crucial component of fungal cell membranes, leading to increased membrane permeability and fungal cell death. *Antibacterial* - **Antibacterial agents** target bacteria, and while some antifungals may have minor antibacterial properties, ketoconazole's primary mechanism and clinical use are not focused on bacteria. - Its mechanism of action, targeting fungal ergosterol synthesis, is distinct from the mechanisms of typical antibacterial drugs. *Antiviral agent* - **Antiviral agents** are specifically designed to inhibit viral replication and are not effective against fungal infections. - Ketoconazole has no significant antiviral activity. *Chemotherapeutic agent* - **Chemotherapeutic agents** are primarily used in the treatment of cancer by targeting rapidly dividing cells. - While some medications can have multiple uses, ketoconazole is not classified or used as a primary chemotherapeutic agent; its main role is in treating fungal infections.

Question 1589: What is the drug of choice in Cryptococcal meningitis?

A. Pentostatin
B. Amphotericin B (Correct Answer)
C. Clotrimazole
D. Zidovudine

Explanation: ***Amphotericin B*** - **Amphotericin B** (in combination with flucytosine) is the **drug of choice** for **induction therapy** in **cryptococcal meningitis** due to its potent fungicidal activity against *Cryptococcus neoformans*. - Its broad-spectrum activity and ability to penetrate the **central nervous system** make it highly effective against severe fungal infections. - Following induction, **fluconazole** is used for consolidation and maintenance therapy. *Pentostatin* - **Pentostatin** is an antineoplastic agent used primarily in the treatment of **hairy cell leukemia**, not fungal infections. - It works by inhibiting **adenosine deaminase**, preventing DNA synthesis in lymphocytes. *Clotrimazole* - **Clotrimazole** is a topical azole antifungal agent used for **superficial fungal infections** like candidiasis or ringworm. - It is not effective for systemic or severe infections like **cryptococcal meningitis** and does not penetrate the CNS adequately. *Zidovudine* - **Zidovudine** (AZT) is an **antiretroviral drug** used in the treatment of **HIV/AIDS**. - It specifically targets the **reverse transcriptase** enzyme of HIV and has no antifungal properties.

Question 1590: Among the following, which one is false about linezolid?

A. It is active against vancomycin resistant enterococci
B. It can be administered orally as well as by i.v. infusion
C. It inhibits bacterial protein synthesis at an early step
D. It is the drug of choice for enterococcal endocarditis (Correct Answer)

Explanation: ***It is the drug of choice for enterococcal endocarditis*** - While linezolid is active against enterococci, it is **not the drug of choice for enterococcal endocarditis**. The standard treatment for **enterococcal endocarditis** is a combination of a penicillin (like ampicillin) or vancomycin with an **aminoglycoside** (like gentamicin) due to synergistic bactericidal activity. - Linezolid is primarily bacteriostatic against enterococci and is reserved for situations where first-line agents are not effective or cannot be used, such as in cases of **vancomycin-resistant enterococci (VRE)**. *It is active against vancomycin-resistant enterococci* - Linezolid is an important antibiotic specifically used for treating infections caused by **vancomycin-resistant enterococci (VRE)**, including *Enterococcus faecium* and *Enterococcus faecalis*. - Its mechanism of action, inhibiting protein synthesis early, allows it to bypass the resistance mechanisms that affect vancomycin. *It can be administered orally as well as by i.v. infusion* - Linezolid has **excellent oral bioavailability**, meaning it is well absorbed from the gastrointestinal tract, making it a valuable option for both oral and intravenous administration. - This characteristic allows for seamless transition from intravenous to oral therapy, facilitating outpatient treatment for certain infections. *It inhibits bacterial protein synthesis at an early step* - Linezolid belongs to the oxazolidinone class of antibiotics and works by specifically binding to the **23S ribosomal RNA** of the 50S subunit. - This binding prevents the formation of the **N-formylmethionyl-tRNA complex**, which is crucial for the initiation of bacterial protein synthesis.

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

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