**Core Concept**
The Revised National Tuberculosis Control Programme (RNTCP) in India has specific guidelines for diagnosing and managing tuberculosis (TB). Multi-drug resistance (MDR) TB is a significant concern, requiring early detection and treatment. The definition of MDR TB is crucial for implementing effective treatment strategies.

**Why the Correct Answer is Right**
MDR TB is defined as resistance to at least isoniazid (H) and rifampicin (R), which are the two most potent first-line antitubercular drugs. This combination is critical because rifampicin is bactericidal, and isoniazid is bacteriostatic. Resistance to both drugs makes TB treatment more challenging and increases the risk of treatment failure and transmission. The WHO and RNTCP recommend a second-line regimen for MDR TB, which includes drugs like streptomycin, ethambutol, and fluoroquinolones.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify resistance to rifampicin, which is a critical component of MDR TB definition.
* **Option B:** This option is incorrect because it does not include resistance to isoniazid, which is also essential for defining MDR TB.
* **Option D:** This option is incorrect because it specifies resistance to a single drug, which does not meet the criteria for MDR TB.

**Clinical Pearl / High-Yield Fact**
Remember that MDR TB requires a second-line regimen, which includes drugs that are more toxic and have more side effects than first-line drugs. It is essential to identify MDR TB early and initiate treatment promptly to prevent further transmission and morbidity.

**Correct Answer:** C. Isoniazid and Rifampicin.