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
Filtration fraction can be best determined by: 
Select and explain. 
A. Inulin and mannitol, B. PAH and Inulin, C. Inulin and creatinine, D. PAH and mannitol
B: PAH and Inulin
Answer: B: PAH and Inulin 
Explanation: To calculate the filtration fraction, which is the fraction of plasma that filters through the glomerular membrane, one must first know the RPF (PAH clearance) and the GFR (inulin clearance). FILTRATION FRACTION = GFR(inulin clearance)/RPF(PAH clearance) Inulin and mannitol measure ECF volume Inulin and creatinine both measures GFR, but creatinine is secreted in the tubules hence GFR estimated by creatinine is falsely higher.