## **Core Concept**
MDR TB, or multidrug-resistant tuberculosis, is a form of tuberculosis infection caused by bacteria that are resistant to treatment with at least two of the most powerful first-line anti-TB drugs: **isoniazid (INH)** and **rifampicin**. This resistance makes the standard six-month treatment regimen ineffective, requiring longer treatment periods with more toxic and expensive drugs.

## **Why the Correct Answer is Right**
The correct answer defines MDR TB as resistance to at least both **isoniazid (INH)** and **rifampicin**, which are the two most potent first-line drugs used in the treatment of tuberculosis. This dual resistance poses significant challenges for TB control programs due to the need for longer treatment durations, higher costs, and increased toxicity of second-line drugs.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Resistance to only one of the first-line drugs does not classify as MDR TB. Resistance to either isoniazid or rifampicin alone is concerning but does not meet the criteria for MDR TB.
- **Option B:** This option might suggest resistance to drugs other than isoniazid and rifampicin, which does not fit the definition of MDR TB. Resistance to other drugs can lead to the classification of extensively drug-resistant TB (XDR TB) or other forms of drug-resistant TB but does not define MDR TB.
- **Option C:** While not provided, any option suggesting resistance to less than two of the key first-line drugs (INH and rifampicin) would be incorrect.

## **Clinical Pearl / High-Yield Fact**
A critical point to remember is that **MDR TB treatment** requires a combination of second-line drugs, which are less effective, more toxic, and much more expensive than first-line drugs. The treatment duration for MDR TB can extend up to 20 months or more, highlighting the importance of adherence and the need for strict infection control measures to prevent transmission.

## **Correct Answer:** C. Resistance to at least both Isoniazid & Rifampicin.