Antimicrobial Agents Practice Questions

Q: Which drug inhibits the enzyme folate synthase?

Sulfonamide. ### Explanation **Correct Option: A. Sulfonamide** Sulfonamides are structural analogs of **Para-Amino Benzoic Acid (PABA)**. They act as competitive inhibitors of the enzyme **Dihydropteroate Synthase** (often referred to broadly as folate synthase). In the bacterial folic acid synthesis pathway, this enzyme is responsible for condensing PABA with pteridine to form dihydropteroic acid. Since bacteria must synthesize their own folate (unlike humans who absorb it from diet), this inhibition leads to a deficiency of tetrahydrofolate, halting DNA synthesis and resulting in a **bacteriostatic** effect. **Incorrect Options:** * **B. Clindamycin:** This is a Lincosamide that inhibits protein synthesis by binding to the **50S ribosomal subunit**, specifically interfering with transpeptidation. * **C. Rifampicin:** This drug inhibits **DNA-dependent RNA polymerase**, thereby blocking bacterial transcription (mRNA synthesis). It is a cornerstone of anti-tubercular therapy. * **D. Norfloxacin:** This is a Fluoroquinolone that inhibits **DNA Gyrase (Topoisomerase II)** and Topoisomerase IV, preventing DNA supercoiling and replication. **NEET-PG High-Yield Pearls:** * **Sequential Blockade:** Trimethoprim inhibits the next step in the pathway (**Dihydrofolate Reductase**). Combining a Sulfonamide with Trimethoprim (e.g., Cotrimoxazole) results in synergistic bactericidal action. * **Resistance Mechanism:** Bacteria most commonly develop resistance to sulfonamides by increasing PABA production or mutating the dihydropteroate synthase enzyme. * **Adverse Effects:** Watch for **Stevens-Johnson Syndrome (SJS)**, Kernicterus in newborns, and crystalluria (prevented by alkalinizing urine).

Q: What is the DOC for management of visceral Leishmaniasis?

Liposomal Amphotericin B. **Liposomal Amphotericin B** is currently the **Drug of Choice (DOC)** for Visceral Leishmaniasis (Kala-azar), especially in the Indian subcontinent [1]. The underlying medical concept relies on its superior efficacy (cure rates >95%) [1] and significantly lower toxicity compared to conventional agents. The liposomal formulation allows the drug to be specifically taken up by the **reticuloendothelial system (macrophages)** in the liver and spleen—the exact site where *Leishmania donovani* resides—thereby increasing therapeutic index and reducing nephrotoxicity. **Analysis of Incorrect Options:** * **Parenteral Sodium Stibogluconate (SSG):** Formerly the first-line treatment, it is no longer the DOC due to widespread **antimonial resistance** (especially in Bihar, India) [2] and severe side effects like cardiotoxicity and pancreatitis. * **Miltefosine:** This is the first **effective oral drug** for Kala-azar [2]. While highly useful for mass treatment, it is not the primary DOC due to teratogenicity (requires strict contraception) and increasing reports of treatment failure. * **Pentamidine:** Previously used as a second-line agent, it is now rarely used for Leishmaniasis due to significant toxicities, including irreversible diabetes mellitus and hypotension. **High-Yield Clinical Pearls for NEET-PG:** * **WHO Protocol:** A single dose of Liposomal Amphotericin B (10 mg/kg) is the preferred regimen in India. * **Post-Kala-azar Dermal Leishmaniasis (PKDL):** The DOC is also Miltefosine (for 12 weeks) or Amphotericin B. * **Mechanism of Amphotericin B:** It binds to **ergosterol** in the fungal/protozoal cell membrane, creating pores that lead to ion leakage and cell death.

Q: Which of the following drugs is likely to damage the eighth cranial nerve when administered for a long period of time?

Streptomycin. **Explanation:** The correct answer is **Streptomycin**. Streptomycin is an **Aminoglycoside** antibiotic used as a first-line drug in the treatment of Tuberculosis. **1. Why Streptomycin is correct:** Aminoglycosides are notorious for their **ototoxicity** and nephrotoxicity. They accumulate in the endolymph and perilymph of the inner ear, leading to the destruction of sensory hair cells. Specifically, Streptomycin is more **vestibulotoxic** than cochleotoxic, causing symptoms like vertigo, tinnitus, and loss of equilibrium due to damage to the **eighth cranial nerve (Vestibulocochlear nerve)**. **2. Why other options are incorrect:** * **Ethambutol:** Its primary dose-related toxicity is **Optic Neuritis** (damage to the second cranial nerve), leading to decreased visual acuity and red-green color blindness. * **Isoniazid (INH):** The hallmark side effect is **Peripheral Neuropathy** (due to Vitamin B6 deficiency) and hepatotoxicity. It does not typically affect the cranial nerves. * **Rifampicin:** Known for causing **orange-colored secretions** (urine, sweat, tears) and hepatotoxicity. It is a potent microsomal enzyme inducer but is not ototoxic. **NEET-PG High-Yield Pearls:** * **Mnemonic for Aminoglycoside Ototoxicity:** **S**treptomycin and **G**entamicin are primarily **Vestibulotoxic** (S-G-V), while **A**mikacin and **N**eomycin are primarily **Cochleotoxic** (A-N-C). * Streptomycin is contraindicated in pregnancy as it can cause **fetal ototoxicity** (congenital deafness). * Always monitor renal function (Creatinine) when using aminoglycosides, as nephrotoxicity often precedes or exacerbates ototoxicity.

Q: Nephrolithiasis occurs with the toxicity to which of the following drugs?

Indinavir. ### Explanation **Correct Answer: C. Indinavir** **Mechanism and Pathophysiology:** Indinavir is a Protease Inhibitor (PI) used in the treatment of HIV. The primary reason for nephrolithiasis (kidney stones) with Indinavir is its **poor solubility** in urine. Approximately 20% of the drug is excreted unchanged in the urine. At physiological urinary pH, Indinavir tends to crystallize, leading to the formation of radiolucent stones. This can manifest clinically as renal colic, hematuria, or crystalluria. To mitigate this risk, patients are strictly advised to maintain high fluid intake (at least 1.5 to 2 liters of water daily). **Analysis of Incorrect Options:** * **A. Ritonavir:** While a potent PI, it is primarily known for its role as a "pharmacokinetic booster" (inhibiting CYP3A4) and its side effects of GI distress and perioral paresthesia, rather than nephrolithiasis. * **B. Saquinavir:** This was the first PI developed. Its main adverse effects include GI upset and QT interval prolongation, but it is not associated with renal stone formation. * **D. Nelfinavir:** This drug is unique among PIs as it cannot be boosted by Ritonavir. Its most characteristic side effect is significant diarrhea, not nephrolithiasis. **High-Yield Clinical Pearls for NEET-PG:** * **Indinavir "Signature" Side Effects:** Nephrolithiasis and Hyperbilirubinemia (indirect/unconjugated). * **Protease Inhibitor Class Effects:** Metabolic syndrome (dyslipidemia, insulin resistance/hyperglycemia) and Lipodystrophy (buffalo hump/central obesity). * **Other drugs causing Nephrolithiasis:** Acetazolamide, Sulfonamides, and Triamterene. * **Management:** Indinavir-induced stones are often managed with hydration and temporary drug discontinuation; they are typically not visible on standard X-rays (radiolucent).

Q: Stavudine (Zerit) is prescribed to a client with human immunodeficiency virus seropositive. The nurse observes which of the following most closely while the client is taking the medication?

Gait. **Explanation:** **Stavudine (d4T)** is a Nucleoside Reverse Transcriptase Inhibitor (NRTI) used in the management of HIV [2]. The correct answer is **Gait** because the most significant dose-limiting toxicity of Stavudine is **peripheral neuropathy** [3]. 1. **Why Gait is Correct:** Stavudine frequently causes distal symmetrical peripheral neuropathy, characterized by numbness, tingling, and pain in the feet and hands [3]. As the neuropathy progresses, it leads to sensory loss and muscle weakness, significantly affecting the patient's **gait** and balance. Monitoring the patient's walking pattern is a critical nursing intervention to detect early signs of neurotoxicity. 2. **Why Other Options are Incorrect:** * **Orientation:** While some antiretrovirals (like Efavirenz) cause CNS side effects, Stavudine is not primarily associated with altered consciousness or disorientation. * **Gag Reflex:** Stavudine does not affect the cranial nerves responsible for the gag reflex (CN IX, X). * **Appetite:** While NRTIs can cause GI upset, appetite is not the specific clinical parameter used to monitor for Stavudine's hallmark toxicity (neuropathy). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Stavudine:** Remember the **"P"s**: **P**eripheral neuropathy and **P**ancreatitis [3]. * **Mechanism of Toxicity:** Stavudine (and Didanosine) inhibits **Mitochondrial DNA polymerase-gamma**, leading to mitochondrial dysfunction [1]. This is the underlying cause of both neuropathy and **Lactic Acidosis** (often accompanied by hepatic steatosis) [1]. * **Lipodystrophy:** Stavudine is strongly associated with **lipoatrophy** (loss of subcutaneous fat from the face and limbs), more so than other NRTIs [3]. * **Avoid Combination:** Stavudine should not be combined with **Zidovudine (AZT)** because they compete for the same intracellular phosphorylation pathway (antagonism) [3].

Q: Which among the following is the drug of choice for Clostridium difficile-induced colitis?

Metronidazole. **Explanation:** **Clostridium difficile-induced colitis** (Pseudomembranous colitis) typically occurs due to the overgrowth of *C. difficile* following broad-spectrum antibiotic therapy (most commonly associated with Clindamycin, Fluoroquinolones, and Cephalosporins). **Why Metronidazole is correct:** Historically, **Metronidazole** (oral) has been the first-line drug of choice for mild-to-moderate cases of *C. difficile* infection (CDI) due to its efficacy, low cost, and narrow anaerobic spectrum. It works by forming toxic free radicals that disrupt the DNA of anaerobic bacteria. **Why the other options are incorrect:** * **Gentamicin:** An Aminoglycoside that is ineffective against anaerobes (it requires oxygen for bacterial uptake) and is a known risk factor for inducing CDI. * **Ciprofloxacin:** A Fluoroquinolone that is a common inciting agent for *C. difficile* overgrowth and has no role in its treatment. * **Linezolid:** An Oxazolidinone used primarily for Gram-positive infections like MRSA and VRE; it is not indicated for CDI. **High-Yield Clinical Pearls for NEET-PG:** 1. **Updated Guidelines:** While Metronidazole is the traditional answer for exams, recent **IDSA guidelines** now prefer **Oral Vancomycin** or **Fidaxomicin** as first-line for all episodes (including mild). However, if Vancomycin is unavailable or for mild cases in resource-limited settings, Metronidazole remains the standard answer. 2. **Route of Administration:** Vancomycin must be given **orally** for CDI; intravenous Vancomycin is ineffective as it does not reach the gut lumen. 3. **Fidaxomicin:** A macrocyclic antibiotic with minimal systemic absorption and a lower rate of recurrence compared to Vancomycin. 4. **Life-threatening/Fulminant CDI:** Treated with a combination of high-dose oral Vancomycin and IV Metronidazole.

Question 1

Which drug is not used in chloroquine-resistant malaria?

Accepted Answer

Fluoroquinolones

Answer

Sulfadoxine–pyrimethamine

Answer

Quinine

Answer

Artemisinins

Question 2

Which drug inhibits the enzyme folate synthase?

Accepted Answer

Sulfonamide

Answer

Clindamycin

Answer

Rifampicin

Answer

Norfloxacin

Question 3

What is the DOC for management of visceral Leishmaniasis?

Accepted Answer

Liposomal Amphotericin B

Answer

Parenteral Sodium stibogluconate

Answer

Miltefosine

Answer

Pentamidine

Question 4

How can aminoglycoside toxicity be decreased?

Accepted Answer

Administering the total daily dose once a day

Answer

Administering the total daily dose twice a day

Answer

Administering a low dose once a day

Answer

Administering a low dose twice a day

Question 5

Which of the following drugs is likely to damage the eighth cranial nerve when administered for a long period of time?

Accepted Answer

Streptomycin

Answer

Ethambutol

Answer

Isoniazid

Answer

Rifampicin

Question 6

Nephrolithiasis occurs with the toxicity to which of the following drugs?

Accepted Answer

Indinavir

Answer

Ritonavir

Answer

Saquinavir

Answer

Nalfinavir

Question 7

All are true about Cotrimoxazole, except?

Accepted Answer

Trimethoprim increases absorption of Sulphamethoxazole

Answer

Resistance is delayed due to drugs in combination

Answer

Trimethoprim : Sulphamethoxazole ratio is 1:5

Answer

Similar pharmacokinetics for both components

Question 8

Stavudine (Zerit) is prescribed to a client with human immunodeficiency virus seropositive. The nurse observes which of the following most closely while the client is taking the medication?

Accepted Answer

Gait

Answer

Orientation

Answer

Gag reflex

Answer

Appetite

Question 9

What is the drug of choice for chlamydial cervicitis?

Accepted Answer

Tetracycline

Answer

Septran

Answer

Chloramphenicol

Answer

Erythromycin

Question 10

Which among the following is the drug of choice for Clostridium difficile-induced colitis?

Accepted Answer

Metronidazole

Answer

Gentamicin

Answer

Ciprofloxacin

Answer

Linezolid

Antimicrobial Agents — MCQs

Antimicrobial Agents — MCQs

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