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 factor/mechanism involved in glomerulotubular balance is: 
Select and explain. 
A. signalling by macula densa cells, B. Oncotic pressure in peritubular capillaries, C. Contraction of the mesangial cells, D. Sluggish blood flow in medullary interstitium
B: Oncotic pressure in peritubular capillaries
Answer: B: Oncotic pressure in peritubular capillaries 
Explanation: G-T balance refers to the mechanism by which increase in GFR and filtration of Na+ propoionately increases the Na+ (and water) reabsorption. Thus, a constant fraction (67%) of the filtered Na+ and water is reabsorbed from PCT, despite fluctuations in GFR. Mechanism: ~ Increased GFR (and increased filtered load of Na+& water) a increased protein concentration in the glomerular capillary plasma (as the fluid has been filtered out of this plasma) Protein-rich plasma leaves the glomerular capillaries efferent aerioles and enters the peritubular capillaries Colloid Oncotic pressure will increase in peritubular capillaries a increased the solute and water reabsorption by the PCT. Options A & C are involved in tubuloglomerular feedback. Sluggish blood flow in medullary interstitium is the basis for countercurrent exchange by vasa recta.