This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Excretory System (Kidney, Bladder) Acid-Base Balance, Subject name: Physiology
The substance that is a vasodilator elsewhere but causes vasoconstriction in kidneys (afferent aeriole) is: 
Select and explain. 
A. Nitric oxide, B. Adenosine, C. Bradykinin, D. Endothelin
B: Adenosine
Answer: B: Adenosine 
Explanation: Adenosine has 2 receptors - A1 & A2 A2 vasodilation (elsewhere) A1 vasoconstriction (kidney) Adenosine is a ligand, pa of the tubuloglomerular feedback. Macula densa cells sense the Na+ level in tubular fluids. Excess Na+ reaching macula densa cells Na+ / K+ ATPase pump in basolateral membranes transpo Na & K+ by degrading ATP Adenosine formed as a by product Adenosine acts as a ligand on the A1 receptors of afferent aeriole Constriction of afferent aerioles