In MDR TB, Mycobacterium tuberculosis is resistant to:
**Core Concept**
The core concept being tested is the definition and treatment of Multi-Drug Resistant Tuberculosis (MDR TB), which is a form of tuberculosis that is resistant to multiple drugs. MDR TB is caused by **Mycobacterium tuberculosis** that is resistant to at least two of the most powerful first-line anti-TB drugs.
**Why the Correct Answer is Right**
MDR TB is defined by resistance to the two most effective anti-TB drugs: **isoniazid** and **rifampicin**. This resistance pattern makes treating MDR TB more challenging and requires the use of second-line drugs, which are less effective, more toxic, and more expensive.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while resistance to other drugs can occur, MDR TB specifically refers to resistance to isoniazid and rifampicin.
**Option B:** Incorrect as it does not specify the correct drug combination that defines MDR TB.
**Option C:** Incorrect because it does not align with the standard definition of MDR TB.
**Option D:** Incorrect as the definition of MDR TB is based on resistance to specific first-line drugs, not all drugs.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that MDR TB treatment regimens are complex and must be individualized based on drug susceptibility patterns.
**Correct Answer:** D. isonizid and rifampicin.