What is the mechanism of action of ethambutol?

• A. Inhibition of arabinosyl transferase, impairing synthesis of the mycobacterial cell wall arabinan.
• B. Inhibition of DNA gyrase.
• C. Inhibition of folate synthesis.
• D. Disruption of protein synthesis at 50S.

Answer: (A)

Ethambutol blocks the arabinosyl transferases that assemble the arabinan portion of the mycobacterial cell wall (arabinan in arabinogalactan and related components). Without proper arabinan synthesis, the cell wall is defective, weakening the wall and slowing the growth of actively dividing mycobacteria. This action is specific to the cell wall assembly process, unlike drugs that inhibit DNA replication (DNA gyrase), folate synthesis, or protein synthesis at the 50S subunit. Ethambutol is primarily bacteriostatic and is used in combination therapy, with optic neuritis as a notable potential side effect.