A 25-year-old woman has had progressive dyspnea and fatigue for the past 2 years. On physical examination, she has pedal edema, jugular venous distention, and hepatomegaly. Lung fields are clear on auscultation. Chest CT scan shows right heart enlargement. Cardiac catheterization is performed, and the pulmonary arterial pressure is increased, without gradients across the pulmonic valve, and no shunts are noted. A transbronchial biopsy is performed, and microscopic examination shows plexiform lesions. A mutation in a gene encoding for which of the following is most likely to cause her pulmonary disease?

**Core Concept**
The patient's presentation of progressive dyspnea, fatigue, pedal edema, jugular venous distention, and hepatomegaly, along with clear lung fields and increased pulmonary arterial pressure, suggests a diagnosis of pulmonary arterial hypertension (PAH). The presence of plexiform lesions on transbronchial biopsy is characteristic of a specific subtype of PAH.

**Why the Correct Answer is Right**
Plexiform lesions are a hallmark of idiopathic pulmonary arterial hypertension (IPAH), which is caused by a mutation in the bone morphogenetic protein receptor type 2 (BMPR2) gene. This gene mutation leads to the formation of plexiform lesions, which are complex vascular structures that obstruct blood flow, resulting in increased pulmonary arterial pressure and resistance. The mutation affects the BMPR2 receptor, which plays a crucial role in the regulation of vascular cell proliferation and differentiation.

**Why Each Wrong Option is Incorrect**
**Option A:** A mutation in the transforming growth factor-beta (TGF-β) receptor gene is associated with familial scleroderma, which is a different condition altogether.
**Option B:** A mutation in the endothelin-1 receptor gene is not directly associated with the formation of plexiform lesions in PAH.
**Option C:** A mutation in the prostacyclin receptor gene is not a primary cause of PAH, although prostacyclin receptor agonists are used as a treatment for PAH.

**Clinical Pearl / High-Yield Fact**
BMPR2 mutations are the most common cause of familial PAH, and patients with a family history of PAH should undergo genetic testing for BMPR2 mutations.

**Correct Answer: D. BMPR2**

✓ Correct Answer: B. Bone morphogenetic receptor 2 (BMPR2)