You are answering a USMLE-style medical question. Question: A 45-year-old man comes to his primary-care doctor with his wife for a 4-week history of constipation. The patient reports that his bowel habits have changed from every day to every 3 or 4 days. He also now has to strain to pass stool. On further questioning, his wife has also noticed that he has seemed fatigued, with little interest in going on their usual afternoon walks. Medical history is notable for hypertension and hyperlipidemia, both managed medically. He was last seen for follow-up of these conditions 1 month ago. Physical examination is unremarkable. Which of the following is the cellular target of the medicine that is most likely responsible for this patient's symptoms? Options: A) Angiotensin-II receptors in the membranes of vascular smooth muscle cells B) Angiotensin-converting enzyme C) Na+/Cl- cotransporter in the membranes of epithelial cells in the distal convoluted tubule D) Na+/K+/2Cl- transporters in the membranes of epithelial cells in the ascending loop of Henle Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.