Problem 10 - Entrance Test

The ability to produce highly concentrated urine is crucial for water conservation and depends on the countercurrent multiplier mechanism, which establishes and maintains a high osmotic gradient in the renal medulla. This gradient is essential for water reabsorption from the collecting ducts under the influence of ADH. Let's break down the components: * Descending Limb of Loop of Henle: Permeable to water, impermeable to solutes. Water leaves, concentrating the filtrate. * Ascending Limb of Loop of Henle: Impermeable to water, actively transports Na+, K+, and Cl- (via the Na+/K+/2Cl- cotransporter) out of the filtrate into the interstitial fluid of the medulla, making the filtrate dilute and building the medullary osmotic gradient. * Vasa Recta: Maintains the medullary osmotic gradient by countercurrent exchange. * Collecting Duct: Permeability to water is regulated by ADH. Water leaves by osmosis, concentrating the urine, using the medullary osmotic gradient. Let's evaluate the options: A. Increased ADH secretion would increase water reabsorption in the collecting duct, leading to more concentrated urine, not an inability to produce it. ADH primarily affects the collecting ducts and distal tubules, not the proximal convoluted tubule (PCT). The PCT reabsorbs most filtered water and solutes regardless of ADH. B. Dysfunction of the Na+/K+/2Cl- cotransporter (NKCC2) in the ascending limb of the loop of Henle: This transporter is critical for actively pumping solutes (Na+, K+, Cl-) out of the filtrate and into the medullary interstitium. If this transporter is impaired (e.g., by loop diuretics), the ascending limb cannot effectively reabsorb these solutes without water, thus failing to create or maintain the high osmotic gradient in the renal medulla. Without this gradient, water cannot be drawn out of the collecting ducts, even if ADH is present, leading to the production of dilute urine. This is a direct cause of inability to concentrate urine. C. Increased reabsorption of water in the descending limb of the loop of Henle: The descending limb is always permeable to water. More water reabsorption here would actually make the filtrate more concentrated as it enters the ascending limb, which would theoretically help with the osmotic gradient, not hinder concentrated urine production. D. Impaired secretion of urea into the collecting duct: Urea recycling (movement from collecting duct into the medulla, then some reabsorption) is important for maintaining the medullary osmotic gradient, especially at its deepest parts. Impaired secretion into the collecting duct (implying less reabsorption in deeper parts and thus less contribution to medullary osmolarity) could slightly reduce the gradient, but the primary driver is the NKCC2 in the ascending limb. Also, the osmotic gradient is primarily in the renal medulla, not the cortex. Therefore, the dysfunction of the NKCC2 cotransporter in the ascending limb is the most critical factor for an inability to produce highly concentrated urine.