This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General physiology, Subject name: Physiology
Free water clearance by the kidney is increased by which of the following 
Select and explain. 
A. Diabetes insipidus, B. Renal failure, C. Diuretic therapy, D. Diabetes mellitus
A: Diabetes insipidus
Answer: A: Diabetes insipidus 
Explanation: Free-water clearance (CH2O) is calculated as the difference between water excretion (urine flow rate) and osmolar clearance.Thus, the rate of free-water clearance represents the rate at which solute-free water is excreted by the kidneys. When free-water clearance is positive, excess water is being excreted by the kidneys; when free-water clearance is negative, excess solutes are being removed from the blood by the kidneys and water is being conserved.Inappropriate secretion of ADH: Either too much or too little ADH secretion results in abnormal water excretion by the kidneys.In Diabetes insipidus: Because the distal tubular segments cannot reabsorb water in the absence of ADH, It results in the formation of a large volume of dilute urine with urine volumes that can exceed 15 L/day.Ref: Guyton; 13th edition; Chapter 29 Urine Concentration and Dilution