## Core Concept
The World Health Organization (WHO) defines **Multidrug-Resistant Tuberculosis (MDR-TB)** as TB that is resistant to at least **isoniazid and rifampicin**, the two most powerful first-line anti-TB drugs. **Extensively Drug-Resistant Tuberculosis (XDR-TB)** is a type of MDR-TB that is further resistant to **any fluoroquinolone and at least one of three injectable second-line drugs**.

## Why the Correct Answer is Right
According to the WHO, XDR-TB cases account for approximately **10% of MDR-TB cases worldwide**. This indicates that a significant proportion of MDR-TB cases have progressed to XDR-TB, which has limited treatment options and a poorer prognosis.

## Why Each Wrong Option is Incorrect
* **Option A:** This option suggests a much lower percentage than the actual figure, which could underestimate the severity of the XDR-TB problem within MDR-TB cases.
* **Option B:** This option suggests a higher percentage than the actual figure, potentially overestimating the prevalence of XDR-TB among MDR-TB cases.
* **Option D:** This option also suggests a different percentage than the correct one, which could mislead about the actual burden of XDR-TB.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **XDR-TB has a significantly worse prognosis than MDR-TB** due to its resistance to more drugs, making treatment longer, more complex, and less successful. Early detection and appropriate management of MDR-TB are crucial to prevent progression to XDR-TB.

## Correct Answer Line
**Correct Answer: C. 10%**