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
A substance is present in a concentration of 2 mg% in the afferent aeriole and zero mg% in the efferent. True about the substance is: 
Select and explain. 
A. It is freely filtered at the glomerulus, B. Secreted in coical nephron, C. Absorbed in PCT, D. Impermeable in the loop of Henle
A: It is freely filtered at the glomerulus
Answer: A: It is freely filtered at the glomerulus 
Explanation: This is only a hypothetical situation. Filtration fraction is about 20%. It means, of the total plasma passing through the glomerulus per unit time, about 20% is filtered out. If a substance has zero concentration in the efferent aeriole, it means all of that substance was freely and completely filtered out in the plasma that has formed the Glomerular Filtrate.