**Core Concept**
Bacteriostatic drugs inhibit bacterial growth without killing the organism, whereas bactericidal drugs kill bacteria. In tuberculosis treatment, understanding whether a drug is bacteriostatic or bactericidal is crucial for regimen design and preventing resistance.

**Why the Correct Answer is Right**
Ethambutol acts by inhibiting bacterial cell wall synthesis, specifically interfering with arabinogalactan formation in mycobacteria. Unlike rifampicin or isoniazid, which are bactericidal, ethambutol does not kill TB bacteria but instead suppresses their growth. This bacteriostatic effect makes it useful as a backup agent in regimens to prevent resistance development, especially when combined with bactericidal drugs.

**Why Each Wrong Option is Incorrect**
Option A: Isoniazid is a potent **bactericidal** agent that inhibits mycolic acid synthesis and is a first-line drug. It kills actively replicating Mycobacterium tuberculosis.
Option B: Rifampicin is a **bactericidal** drug that inhibits RNA polymerase, leading to rapid killing of bacteria. It is a cornerstone of TB therapy.
Option C: Streptomycin is a **bactericidal** aminoglycoside that disrupts protein synthesis in bacteria and is effective against TB.

**Clinical Pearl / High-Yield Fact**
In TB regimens, **ethambutol** is used as a bacteriostatic agent to complement bactericidal drugs, reducing risk of resistance. Always include it in initial therapy for drug-susceptible TB, especially in monotherapy or when resistance is suspected.

✓ Correct Answer: D. Ethambutol