Which first line antitubercular drug is bacteriostatic ?
**Core Concept**
The question is testing the student's knowledge of the bacteriostatic and bactericidal effects of antitubercular drugs. Bacteriostatic agents inhibit bacterial growth but do not kill the bacteria, whereas bactericidal agents directly kill the bacteria.
**Why the Correct Answer is Right**
Isoniazid is a first-line antitubercular drug that acts by inhibiting the synthesis of mycolic acid, an essential component of the Mycobacterium tuberculosis cell wall. This inhibition prevents the formation of the mycobacterial cell wall, ultimately leading to the death of the bacteria. However, Isoniazid also has bacteriostatic effects, particularly at lower concentrations, by inhibiting the synthesis of fatty acids and mycolic acid. This makes it a unique agent with both bactericidal and bacteriostatic properties.
**Why Each Wrong Option is Incorrect**
* **Option A:** Rifampicin is a bactericidal agent that works by inhibiting RNA synthesis in M. tuberculosis. It is not bacteriostatic.
* **Option B:** Ethambutol is a bacteriostatic agent that inhibits the synthesis of the cell wall by interfering with the arabinosyl transferase enzyme. However, it is not the first-line antitubercular drug that is primarily bacteriostatic.
* **Option C:** Pyrazinamide is a bactericidal agent that works by inhibiting fatty acid synthase I, leading to the death of M. tuberculosis in acidic environments. It is not primarily bacteriostatic.
**Clinical Pearl / High-Yield Fact**
It is essential to note that the bacteriostatic and bactericidal effects of antitubercular drugs can be influenced by the concentration of the drug, the duration of exposure, and the presence of resistance. This highlights the importance of using combination therapy and monitoring drug levels to optimize treatment outcomes.
**Correct Answer: A. Isoniazid**