**Core Concept**
Pneumonectomy, a surgical procedure to remove an entire lung, requires careful control of blood flow to prevent bleeding and ensure safe removal of the lung. The first step in a pneumonectomy involves ligation of the pulmonary artery to prevent massive bleeding from the lung's arterial supply.

**Why the Correct Answer is Right**
The pulmonary artery is the main blood vessel supplying oxygenated blood to the lung. Ligation of the pulmonary artery (Option B) is the first step in a pneumonectomy to prevent catastrophic bleeding from the arterial side. This is a critical step to ensure safe dissection and removal of the lung. The pulmonary artery is ligated before the bronchus is divided to prevent bleeding from the lung tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** Ligation of the pulmonary vein is not the first step in a pneumonectomy. The pulmonary veins are ligated after the pulmonary artery to prevent bleeding from the venous side.

**Option C:** Division of the bronchus is not the first step in a pneumonectomy. The bronchus is divided after the pulmonary artery has been ligated to prevent air from entering the lung tissue and causing further bleeding.

**Option D:** Lymph node clearance is not the first step in a pneumonectomy. While lymph node clearance is an essential part of the surgical procedure, it is performed after the lung has been removed and the pulmonary artery has been ligated.

**Clinical Pearl / High-Yield Fact**
Remember the "PA-PV" sequence in pneumonectomy: pulmonary artery ligation followed by pulmonary vein ligation to prevent bleeding. This sequence is crucial to ensure safe removal of the lung.

**✓ Correct Answer: B. Ligate pulmonary aery**