Notes on Urinary Tract Antiseptics and MCQ for NEETPG, GPAT, NORCET, Medical, Pharmacy and Nursing Exams

Notes on Urinary Tract Antiseptics and MCQ for NEETPG, GPAT, NORCET, Medical, Pharmacy and Nursing Exams

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Urinary tract antiseptics are drugs that exert their effect mainly in the urinary tract and are not used for systemic infections. They are especially useful in lower urinary tract infections (UTIs) like cystitis.

1. Nitrofurantoin

  • Class: Nitrofuran antibiotic

  • Mechanism:

    • Reduced by bacterial flavoproteins to reactive intermediates

    • These intermediates inactivate or alter ribosomal proteins and other macromolecules (DNA, RNA, metabolic enzymes)

  • Spectrum:

    • Active against E. coli, Enterococci, and some Gram-positives

  • Uses:

    • Uncomplicated lower UTIs, especially in women

    • Prophylaxis in recurrent UTIs

  • Side Effects:

    • GI upset

    • Pulmonary toxicity (chronic use)

    • Hepatotoxicity

    • Hemolysis in G6PD deficiency

    • Peripheral neuropathy (long-term use)

  • Contraindication:

    • Renal impairment (CrCl < 60 mL/min)

    • Pregnancy (near term)

2. Fosfomycin

  • Class: Phosphonic acid derivative

  • Mechanism:

    • Inhibits enolpyruvyl transferase, preventing the formation of N-acetylmuramic acid, an essential component of the bacterial cell wall

  • Spectrum:

    • Broad: E. coli, Enterococci, Klebsiella, some MRSA

  • Uses:

    • Single-dose therapy for uncomplicated UTIs in women

  • Side Effects:

    • Diarrhea

    • Headache

    • Rare hypersensitivity

  • Advantages:

    • Single oral dose

    • Low resistance rate

3. Methenamine (Hexamine)

  • Class: Urinary antiseptic

  • Mechanism:

    • Hydrolyzed in acidic urine to release formaldehyde, which is bactericidal

  • Effective pH: <5.5

  • Uses:

    • Chronic suppression of UTIs

    • Prophylaxis in recurrent UTIs

  • Side Effects:

    • GI irritation

    • Hematuria (high doses)

    • Bladder irritation

  • Contraindication:

    • Hepatic dysfunction (formaldehyde toxicity)

    • Renal failure (risk of crystalluria)

  • Often combined with: Acidifiers (e.g., ascorbic acid)

📊 Comparison Chart of Nitrofurantoin Fosfomycin and Methenamine

Feature Nitrofurantoin Fosfomycin Methenamine
Chemical Class Nitrofuran derivative Phosphonic acid derivative Urinary antiseptic (prodrug of formaldehyde)
Mechanism of Action Reduced to reactive intermediates that damage DNA, proteins Inhibits MurA enzyme → blocks cell wall synthesis Hydrolyzed in acidic urine to formaldehyde, bactericidal
Spectrum E. coli, Enterococci, some Gram + and Gram – bacteria Broad: E. coli, Klebsiella, Enterococci, some MRSA Broad (due to non-specific formaldehyde activity)
Uses – Acute uncomplicated UTI (lower tract) – UTI prophylaxis – Single-dose therapy for uncomplicated UTI – Prophylaxis of chronic UTI – Not used for active infection
Side Effects – GI upset – Pulmonary toxicity (chronic) – Hepatotoxicity – Neuropathy – Hemolysis (G6PD deficiency) – GI upset – Headache – Rare rash/hypersensitivity – GI irritation – Hematuria – Bladder irritation
Contraindications – Renal insufficiency (CrCl < 60 mL/min) – Late pregnancy – G6PD deficiency – Severe renal failure (dose adjust) – Hypersensitivity – Renal/hepatic dysfunction – Alkaline urine (ineffective)
Dosing Note Multiple daily doses Single oral dose (3 g sachet) Needs acidic urine to work; often given with acidifiers

🔄 Drug-Drug Interactions

Nitrofurantoin:

  • Antacids (Mg-based): ↓ Absorption

  • Probenecid: ↓ Renal excretion → ↑ Toxicity

  • Quinolones: Antagonistic effect

Fosfomycin:

  • Metoclopramide: ↓ Fosfomycin serum levels

  • Minimal interaction otherwise

Methenamine:

  • Sulfonamides: Risk of crystalluria (form insoluble compounds)

  • Urinary alkalinizers: ↓ Efficacy (needs acidic urine)

  • Ascorbic acid: ↑ Efficacy (urine acidifier)

📝 Multiple Choice Questions (MCQs)

  1. Which urinary antiseptic is contraindicated in renal failure due to crystalluria risk?

    • A. Nitrofurantoin

    • B. Fosfomycin

    • C. Methenamine

    • D. Amoxicillin
      ✅ Answer: C

  2. Which drug acts by inhibiting bacterial cell wall synthesis at the early stage?

    • A. Fosfomycin

    • B. Nitrofurantoin

    • C. Methenamine

    • D. Trimethoprim
      ✅ Answer: A

  3. Which of the following is a common side effect of Nitrofurantoin on chronic use?

    • A. Nephrotoxicity

    • B. Pulmonary fibrosis

    • C. Constipation

    • D. Bradycardia
      ✅ Answer: B

  4. What is the mechanism of action of Methenamine?

    • A. Inhibits DNA gyrase

    • B. Generates formaldehyde in acidic urine

    • C. Inhibits protein synthesis

    • D. Blocks folic acid synthesis
      ✅ Answer: B

  5. Which of the following reduces the urinary antibacterial efficacy of Methenamine?

    • A. Ascorbic acid

    • B. Sulfonamides

    • C. Urinary alkalinizers

    • D. Metoclopramide
      ✅ Answer: C

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