The pulmonary artery is different from the pulmonary vein in several aspects, but based on the options provided, the correct answer is:

Option B: Its wall is thick.

Let's explore why this is the case and clarify the reasoning behind each option:

Option A: Its lumen is broad. Both the pulmonary artery and vein can have broad lumens, but this characteristic does not fundamentally distinguish one from the other. The lumen size can vary based on the specific segment of the vessel and the volume of blood it is transporting.

Option B: Its wall is thick. This is true and distinguishes the pulmonary artery from the pulmonary vein. Typically, arteries have thicker walls than veins because they must withstand and convey the higher pressure of blood pumped from the heart. Even though the pulmonary artery carries deoxygenated blood to the lungs — and does so at a lower pressure than systemic arteries — its walls are still thicker than those of the pulmonary veins, which carry oxygenated blood back to the heart.

Option C: It has valves. This option is not correct. Pulmonary arteries do not have valves. The confusion might arise from the fact that vein systems, including pulmonary veins, generally have valves to prevent backflow of blood. This characteristic is not found in the pulmonary artery.

Option D: It does not possess endothelium. This is incorrect. All blood vessels, including the pulmonary artery and pulmonary vein, are lined with endothelium. The endothelium is a thin layer of cells that provides a barrier between the blood and the rest of the vessel wall, facilitating smooth blood flow and playing roles in vascular health and disease prevention.

Therefore, the primary distinction between the pulmonary artery and the pulmonary vein, as highlighted by the options given, is the thickness of their walls (Option B).