Drug affecting positive free water clearance without affecting negative free water clearance -

Ans. is 'b' i.e., Thiazides o Loop diuretics abolishes the corticomedullary osmotic gradient and blocks positive as well as negative free water clearance. (Free water clearance is the volume of urine excreted per unit time in excess of that required to excrete the contained solute isoosmotically with plasma. It is positive when dilute urine is passed, negative when concentrated urine is passed and zero when isotonic urine is passed)o Thiazides decrease positive free water clearance without affecting negative free water clearance.