**Core Concept**
XDR-TB (Extensively Drug-Resistant Tuberculosis) is a form of tuberculosis that is resistant to the most effective first-line and second-line anti-TB medications. This condition poses a significant challenge in TB treatment and management.

**Why the Correct Answer is Right**
The definition of XDR-TB involves resistance to at least four of the core anti-TB drugs: isoniazid, rifampicin, any fluoroquinolone, and at least one of three injectable second-line drugs (i.e., amikacin, kanamycin, or capreomycin). This resistance pattern makes XDR-TB treatment more complex and difficult to cure. The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) have established this definition to guide TB treatment and surveillance efforts.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the required number of drug resistances for XDR-TB.
**Option B:** This option is incorrect because it only mentions resistance to first-line drugs, which is characteristic of MDR-TB (Multi-Drug Resistant Tuberculosis), not XDR-TB.
**Option C:** This option is incorrect because it does not include the injectable second-line drugs that are essential for defining XDR-TB.

**Clinical Pearl / High-Yield Fact**
It's essential to note that XDR-TB is a subset of MDR-TB, and its diagnosis requires specialized laboratory testing and expertise. Prompt identification and management of XDR-TB are crucial to prevent its spread and improve patient outcomes.

**Correct Answer:** D.