This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Gynaecology & Obstetrics
[{'content': 'Keyhole sign is seen in \nSelect and explain. \nA. Polycystic kidney disease, B. Renal agenesis, C. Ureteropelvic junction obstruction, D. Bladder outlet obstruction', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Distal obstruction of the urinary tract is more frequent in male fetuses, and the most common etiology is posterior urethral valves. Characteristically, there is dilatation of the bladder and proximal urethra, termed the"keyhole" sign, and the bladder wall is thick (Ref: William\'s Obstetrics; 25th edition)', 'role': 'assistant'}]