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
The tubuloglomerular feedback is me­diated by: 
Select and explain. 
A. Sensing of Na+  concentration in the macula densa, B. Sensing of HCO3 - concentration in macula densa, C. Sensing NaCl concentration in the macula densa, D. Opening up of voltage gated Na+ channels in afferent arteriole
C: Sensing NaCl concentration in the macula densa
Answer: C: Sensing NaCl concentration in the macula densa 
Explanation: Macula densa sense both Na+ and Cl- , but Cl- is more important.