This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory System Pa 1, Subject name: Physiology
Most potent vasoconstrictor of the pulmonary aery is: 
Select and explain. 
A. Endothelin, B. Hypoxia, C. Angiotensin, D. TXA2
B: Hypoxia
Answer: B: Hypoxia 
Explanation: Endothelin, urotensin, angiotensin are the potent constrictors of the systemic vessels. In case of the pulmonary vessels, hypoxia is the strongest vasoconstrictor, acting the O2-sensitive K+ channels in the pulmonary vascular smooth muscle cells (VSMCs).