**Core Concept**
Extensively Drug-Resistant Tuberculosis (XDR-TB) is a type of multidrug-resistant tuberculosis (MDR-TB) that is resistant to at least four of the core second-line anti-TB drugs, in addition to the core first-line anti-TB drugs: isoniazid and rifampicin.

**Why the Correct Answer is Right**
XDR-TB is a serious form of TB that is difficult to treat and requires a complex regimen of second-line anti-TB drugs. The resistance to these drugs is often due to mutations in the genes encoding the target enzymes of the drugs, such as the rpoB gene for rifampicin and the katG gene for isoniazid. The use of second-line anti-TB drugs, such as amikacin, kanamycin, and moxifloxacin, is essential for the treatment of XDR-TB.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because isoniazid resistance is a characteristic of MDR-TB, not XDR-TB.
**Option B:** This option is incorrect because rifampicin resistance is a characteristic of MDR-TB, not XDR-TB.
**Option C:** This option is incorrect because the definition of XDR-TB specifically excludes resistance to the fluoroquinolones, such as ofloxacin, which is used as a second-line anti-TB drug.

**Clinical Pearl / High-Yield Fact**
The World Health Organization (WHO) has developed a standardized definition of XDR-TB, which includes resistance to at least four of the core second-line anti-TB drugs, in addition to the core first-line anti-TB drugs.

**Correct Answer:** B.