This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Thromboxane A2 is secreted by which of the following cells? 
Select and explain. 
A. Macrophages, B. Platelets, C. Smooth muscle cells, D. Reticuloendothelial cells
B: Platelets
Answer: B: Platelets 
Explanation: Thromboxane A2 is synthesized by platelets and promotes vasoconstriction and platelet aggregation. Prostacyclin is produced by endothelial and smooth muscle cells in the walls of blood vessels and inhibit platelet aggregation while promoting vasodilation.