Ethambutol – mechanism of action MCQs With Answer

Ethambutol – mechanism of action MCQs With Answer

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Ethambutol – mechanism of action MCQs With Answer

Ethambutol is a first-line antitubercular agent important for B.Pharm students to understand, particularly its mechanism of action and clinical implications. It is a bacteriostatic inhibitor of mycobacterial cell wall biosynthesis that targets arabinosyl transferases (Emb proteins), blocking arabinogalactan polymerization and weakening the mycobacterial cell envelope. Key topics include dosing, pharmacokinetics, renal excretion, resistance mechanisms (embB mutations), and dose-related ocular toxicity such as optic neuritis and color vision defects. Familiarity with monitoring (visual acuity, Ishihara plates), dose adjustment in renal impairment, and its role in combination therapy (RIPE regimen) is essential.

Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the primary mechanism of action of ethambutol against Mycobacterium tuberculosis?

  • Inhibition of mycolic acid synthesis by targeting FAS-II enzymes
  • Inhibition of arabinosyl transferase enzymes blocking arabinogalactan polymerization
  • Disruption of DNA gyrase leading to DNA strand breaks
  • Inhibition of protein synthesis at the 30S ribosomal subunit

Correct Answer: Inhibition of arabinosyl transferase enzymes blocking arabinogalactan polymerization

Q2. Ethambutol is classified pharmacodynamically as which of the following against M. tuberculosis?

  • Bactericidal
  • Bacteriostatic
  • Fungistatic
  • Virucidal

Correct Answer: Bacteriostatic

Q3. Which cell wall component synthesis is directly affected by ethambutol?

  • Peptidoglycan cross-linking
  • Arabinogalactan polymerization
  • Mycolic acid condensation
  • Lipooligosaccharide formation

Correct Answer: Arabinogalactan polymerization

Q4. Mutations in which gene are most commonly associated with ethambutol resistance?

  • katG
  • rpoB
  • embB
  • gyrA

Correct Answer: embB

Q5. What is the usual recommended adult dose of ethambutol in daily tuberculosis therapy?

  • 5 mg/kg once daily
  • 15 mg/kg once daily
  • 50 mg/kg once daily
  • 100 mg once weekly

Correct Answer: 15 mg/kg once daily

Q6. Which of the following is the most clinically important adverse effect of ethambutol?

  • Hepatotoxicity with transaminase elevation
  • Severe neutropenia
  • Optic neuritis with decreased visual acuity and color vision defects
  • Ototoxicity with hearing loss

Correct Answer: Optic neuritis with decreased visual acuity and color vision defects

Q7. What baseline and periodic monitoring is essential for patients on ethambutol?

  • Liver function tests monthly
  • Serum creatinine and electrolytes weekly
  • Visual acuity and color vision testing at baseline and monthly
  • Complete blood count daily

Correct Answer: Visual acuity and color vision testing at baseline and monthly

Q8. How is ethambutol primarily eliminated from the body?

  • Hepatic metabolism followed by biliary excretion
  • Renal excretion, largely unchanged
  • Respiratory elimination via exhaled air
  • Metabolism to active metabolites in the gut

Correct Answer: Renal excretion, largely unchanged

Q9. What adjustment is required for ethambutol in severe renal impairment?

  • No adjustment needed; give standard dose
  • Increase dose frequency to twice daily
  • Reduce dose or extend dosing interval due to decreased clearance
  • Switch to intravenous formulation

Correct Answer: Reduce dose or extend dosing interval due to decreased clearance

Q10. Which visual defect is classically associated with ethambutol toxicity?

  • Blue-yellow color blindness
  • Central scotoma without color change
  • Red-green color vision impairment
  • Peripheral tunnel vision only

Correct Answer: Red-green color vision impairment

Q11. Ethambutol is commonly included in which first-line anti-TB regimen acronym?

  • MAZE regimen
  • RIPE regimen
  • PARM regimen
  • DOTS-Plus regimen only

Correct Answer: RIPE regimen

Q12. Which Emb proteins are primary molecular targets of ethambutol?

  • EmbX and EmbY
  • EmbA, EmbB and EmbC
  • Emb1 and Emb2 only
  • EmbM and EmbN

Correct Answer: EmbA, EmbB and EmbC

Q13. Which diagnostic test is most appropriate for routine color vision monitoring in patients on ethambutol?

  • Snellen chart only
  • Ishihara color plates
  • Electroretinogram (ERG) monthly
  • Visual evoked potentials weekly

Correct Answer: Ishihara color plates

Q14. In the mechanism of ethambutol, inhibition of arabinosyl transferases primarily interferes with which linkage in the mycobacterial cell envelope?

  • Peptidoglycan-peptide crosslinking
  • Attachment and polymerization of arabinogalactan to the cell wall
  • Synthesis of trehalose dimycolate
  • Glycolipid transport across membrane

Correct Answer: Attachment and polymerization of arabinogalactan to the cell wall

Q15. The most common molecular mechanism by which Mycobacterium acquires resistance to ethambutol is:

  • Overexpression of efflux pumps only
  • Mutation in embB leading to altered arabinosyl transferase
  • Plasmid-mediated enzymatic inactivation
  • Increased cell wall permeability

Correct Answer: Mutation in embB leading to altered arabinosyl transferase

Q16. Which statement about ethambutol pharmacokinetics is correct?

  • It is extensively metabolized by CYP3A4 into active metabolites
  • It has poor oral absorption and must be given IV
  • It is well absorbed orally and largely excreted unchanged by kidneys
  • It is secreted into bile and reabsorbed enterohepatically

Correct Answer: It is well absorbed orally and largely excreted unchanged by kidneys

Q17. Which of the following statements about ethambutol use in pregnancy is most accurate?

  • Contraindicated due to teratogenicity
  • Generally considered safe and can be used if benefit outweighs risk
  • Requires routine fetal vision screening during therapy
  • Should be replaced by streptomycin in all pregnant patients

Correct Answer: Generally considered safe and can be used if benefit outweighs risk

Q18. Which of the following clinical signs would most likely prompt immediate discontinuation of ethambutol?

  • Transient mild nausea
  • Significant new decrease in visual acuity and color vision
  • Elevated ALT twice the normal limit with no symptoms
  • Mild peripheral neuropathy after one dose

Correct Answer: Significant new decrease in visual acuity and color vision

Q19. Ethambutol’s antibacterial spectrum primarily includes:

  • Typical Gram-positive cocci only
  • Many Gram-negative rods but not mycobacteria
  • Mycobacteria including M. tuberculosis and some atypical mycobacteria
  • Fungi and certain protozoa

Correct Answer: Mycobacteria including M. tuberculosis and some atypical mycobacteria

Q20. Which laboratory method is commonly used to measure plasma ethambutol concentrations in research or therapeutic monitoring?

  • High-performance liquid chromatography (HPLC)
  • Complete blood count
  • Western blot assay
  • Microscopy with Ziehl-Neelsen stain

Correct Answer: High-performance liquid chromatography (HPLC)

Q21. Risk of ethambutol-induced optic neuritis is most strongly correlated with which factor?

  • Concurrent use of pyrazinamide
  • Higher daily doses and prolonged duration
  • Patient age under 5 years only
  • Administration with food

Correct Answer: Higher daily doses and prolonged duration

Q22. How should ethambutol be dosed in pediatric patients?

  • Same flat adult dose irrespective of weight
  • Weight-based dosing, commonly 15–25 mg/kg once daily with careful monitoring
  • Only once weekly dosing at 100 mg/kg
  • Not recommended at any pediatric age

Correct Answer: Weight-based dosing, commonly 15–25 mg/kg once daily with careful monitoring

Q23. Which organ system is the primary target of ethambutol toxicity?

  • Hepatic system
  • Renal glomeruli
  • Visual/optic nerve system
  • Cardiac conduction system

Correct Answer: Visual/optic nerve system

Q24. When ethambutol-induced optic neuritis is suspected, the immediate management should be:

  • Reduce the dose by half and continue therapy
  • Stop ethambutol immediately and perform ophthalmologic evaluation
  • Ignore mild symptoms and continue treatment for full course
  • Switch to a fluoroquinolone without further testing

Correct Answer: Stop ethambutol immediately and perform ophthalmologic evaluation

Q25. Ethambutol’s inclusion in combination TB therapy primarily helps to:

  • Prevent hepatotoxicity caused by isoniazid
  • Reduce the emergence of resistance by targeting a different cell wall pathway
  • Act as the sole sterilizing agent against dormant bacilli
  • Provide rapid symptomatic relief of cough

Correct Answer: Reduce the emergence of resistance by targeting a different cell wall pathway

Q26. Which of the following decreases ethambutol absorption when administered concomitantly?

  • Strong acidic beverages
  • Antacids containing aluminum or magnesium
  • Vitamin C supplements
  • Proton pump inhibitors increase absorption markedly

Correct Answer: Antacids containing aluminum or magnesium

Q27. Ethambutol’s half-life in patients with normal renal function is approximately:

  • 30 minutes
  • 3–4 hours
  • 24–48 hours
  • 7–10 days

Correct Answer: 3–4 hours

Q28. Which clinical feature differentiates ethambutol optic neuritis from other ocular conditions?

  • Predominant blue-yellow dyschromatopsia only
  • Painless progressive loss of central vision with red-green dyschromatopsia
  • Severe eye pain with photophobia
  • Acute proptosis and conjunctival hemorrhage

Correct Answer: Painless progressive loss of central vision with red-green dyschromatopsia

Q29. In the lab, ethambutol-resistant M. tuberculosis is most likely to show which phenotypic change?

  • Increased susceptibility to isoniazid
  • Growth on media containing ethambutol concentrations that inhibit wild-type strains
  • Loss of acid-fast staining properties
  • Complete inability to grow on synthetic media

Correct Answer: Growth on media containing ethambutol concentrations that inhibit wild-type strains

Q30. For a patient who develops mild reversible ethambutol optic neuritis after 4 weeks, what is the expected course after stopping the drug?

  • Progressive irreversible blindness despite stopping drug
  • Gradual improvement of vision over weeks to months in many cases
  • Immediate complete recovery within 24 hours
  • No change; vision remains the same permanently

Correct Answer: Gradual improvement of vision over weeks to months in many cases

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