**Core Concept**
Pulmonary hypertension (PH) is a complex condition characterized by elevated pulmonary artery pressure, leading to right heart failure and potentially life-threatening complications. The definition of PH is crucial for accurate diagnosis and management.

**Why the Correct Answer is Right**
The correct answer is based on the World Health Organization (WHO) classification, which defines PH as a mean pulmonary artery pressure (mPAP) ≥ 25 mmHg at rest, as measured by right heart catheterization. This measurement is essential for diagnosing PH, which can be further classified into five groups based on the underlying cause and hemodynamic characteristics. The mPAP value of 25 mmHg is critical, as it distinguishes PH from normal pulmonary artery pressure, which is typically < 20 mmHg. **Why Each Wrong Option is Incorrect** **Option A:** Incorrect because a mPAP value of 20 mmHg is considered normal, not indicative of PH. **Option B:** Incorrect as it is too low to be considered PH. **Option C:** Incorrect as it is a value that may be seen in some cases of PH, but not a definitive criterion for diagnosis. **Clinical Pearl / High-Yield Fact** It is essential to remember that PH can be diagnosed using various non-invasive tests, such as echocardiography and ventilation-perfusion scans, but right heart catheterization remains the gold standard for measuring mPAP and diagnosing PH. **Correct Answer:** D. 25 mmHg.