This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Blood Physiology, Subject name: Physiology
Erythropoietin is secreted by: 
Select and explain. 
A. Coical interstitial cells, B. Cells in macula densa, C. Granular cells, D. Mesangial cells
A: Coical interstitial cells
Answer: A: Coical interstitial cells 
Explanation: JG apparatus Cells in macula densa Granular cells/ JG cells Mesangial cells Senses Na+ & Cl- in tubular fluids & initiate tubuloglomerular feedback. Synthesis & secrete renin Cells that can alternate the vessel diameter. Type I Coical interstitial cells : synthesize erythropoietin