This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Oncology, Subject name: Medicine
A 78-year-old man is brought to the hospital because of persistent nausea and vomiting. On examination he appears dry, his abdomen is soft, and the JVP is not visible. Laboratory investigations reveal hypernatremia (158 mEq/L) and his calculated free water deficit is approximately 3 L. What segment of the normal kidney is most of the water reabsorbed from? 
Select and explain. 
A. collecting ducts, B. proximal tubule, C. distal tubule, D. ascending loop of Henle
B: proximal tubule
Answer: B: proximal tubule 
Explanation: The largest volume of water is reabsorbed in the nephron at the proximal convolution. Maximally concentrated urine depends on ADH, which allows distal convoluted tubes and collecting ducts to become permeable to water.