You are answering a USMLE-style medical question. Question: A 17-year-old girl is brought to the physician because she has not attained menarche. There is no personal or family history of serious illness. She is 168 cm (5 ft 5 in) tall and weighs 63 kg (139 lb); BMI is 22.3 kg/m2. Examination shows normal breast development. Scant axillary hair is present. Abdominal examination shows a firm, nontender left inguinal mass. Pelvic examination shows a blind vaginal pouch. Ultrasonography does not show a uterus or ovaries. Which of the following is the most likely underlying cause of this patient's symptoms? Options: A) 5-α reductase deficiency B) Sex chromosome mosaicism C) Sex chromosome monosomy D) Androgen insensitivity Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.