You are answering a USMLE-style medical question. Question: A 32-year-old primigravid woman at 16 weeks' gestation comes to the physician for a routine prenatal visit. She is asymptomatic and has no history of serious illness. Her only medication is a prenatal vitamin. Her temperature is 37.2°C (99°F) and blood pressure is 108/60 mm Hg. Pelvic examination shows a uterus consistent in size with a 16-week gestation. A quadruple screening test shows maternal serum AFP of 3 times the median and normal levels of serum β-hCG, estriol, and inhibin A. Which of the following is most likely to account for these findings? Options: A) Partial molar pregnancy B) Neural tube defect C) Trisomy 18 D) Trisomy 21 Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.