You are answering a USMLE-style medical question. Question: A 31-year-old woman comes to the physician because of a 5-month history of intermittent flank pain. Over the past 2 years, she has had five urinary tract infections. Her blood pressure is 150/88 mm Hg. Physical examination shows bilateral, nontender upper abdominal masses. Serum studies show a urea nitrogen concentration of 29 mg/dL and a creatinine concentration of 1.4 mg/dL. Renal ultrasonography shows bilaterally enlarged kidneys with multiple parenchymal anechoic masses. Which of the following is the most likely diagnosis? Options: A) Medullary sponge kidney B) Autosomal dominant polycystic kidney disease C) Autosomal recessive polycystic kidney disease D) Obstructive cystic dysplasia Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.