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
Angiotensin-2 mainly act by - 
Select and explain. 
A. Afferent aeriole dilatation, B. Efferent aeriole dilatation, C. Efferent aeriole constriction, D. Afferent aeriole constriction
C: Efferent aeriole constriction
Answer: C: Efferent aeriole constriction 
Explanation: Angiotensin-II has several effects that make the kidneys retain salt and water, It constricts the efferent aerioles, which has two effects on peritubular capillary dynamics that increase sodium and water reabsorption. Efferent aeriolar constriction reduces peritubular capillary hydrostatic pressure, which increases net tubular reabsorption, especially from the proximal tubules. Efferent aeriolar constriction, by reducing renal blood flow, raises filtration fraction in the glomerulus and increases the concentration of proteins and the colloid osmotic pressure in peritubular capillaries, This mechanism increases the resorptive force at the peritubular capillaries and raises tubular reabsorption of sodium and water.