Which anti TB drug cause resistance fastest?
**Core Concept**
The question is testing the student's knowledge of the mechanism of resistance development in anti-tuberculosis (TB) drugs. The correct answer requires an understanding of the pharmacodynamics and pharmacokinetics of these drugs, as well as the genetic factors that contribute to resistance.
**Why the Correct Answer is Right**
Isoniazid (INH) is a key first-line anti-TB drug that targets the enzyme InhA, which is involved in the synthesis of mycolic acids in the Mycobacterium tuberculosis cell wall. INH works by inhibiting the activity of InhA, leading to the accumulation of toxic fatty acid intermediates and ultimately, cell death. However, M. tuberculosis can develop resistance to INH through mutations in the inhA gene or the katG gene, which encodes the enzyme responsible for activating INH. The katG gene mutation can lead to the production of a non-functional enzyme, resulting in a 10- to 100-fold increase in the minimum inhibitory concentration (MIC) of INH. This rapid development of resistance is due to the high mutation rate of the katG gene and the fact that INH is a prodrug that requires activation by the KatG enzyme.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because streptomycin (SM) resistance is primarily due to mutations in the rpsL gene, which encodes the ribosomal protein S12. While SM resistance can develop quickly, it is not as rapid as INH resistance.
* **Option B:** This option is incorrect because rifampicin (RIF) resistance is primarily due to mutations in the rpoB gene, which encodes the beta subunit of the RNA polymerase. While RIF resistance can develop relatively quickly, it is not as rapid as INH resistance.
* **Option C:** This option is incorrect because ethambutol (EMB) resistance is primarily due to mutations in the embB gene, which encodes the arabinosyl transferase involved in the synthesis of the Mycobacterium tuberculosis cell wall. While EMB resistance can develop relatively quickly, it is not as rapid as INH resistance.
**Clinical Pearl / High-Yield Fact**
Remember that the rapid development of INH resistance is due to the high mutation rate of the katG gene and the fact that INH is a prodrug that requires activation by the KatG enzyme. This highlights the importance of using a combination of anti-TB drugs to prevent the development of resistance.
**Correct Answer: A. Isoniazid (INH)**