You are answering a USMLE-style medical question. Question: A 25-year-old African American man presents to his primary care provider for routine blood work. He is a well-known case of sickle cell disease treated with hydroxyurea and tramadol as needed for pain. He does not smoke or drink alcohol. The patient asks if his disease can adversely affect his kidneys because his friend was recently diagnosed with end-stage renal failure. Which of the following is a renal complication of sickle cell disease? Options: A) Membranous glomerulonephritis B) Proximal renal tubular acidosis C) Impaired urine diluting ability D) Impaired urine concentration ability Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.