You are answering a USMLE-style medical question. Question: A 27-year-old woman, gravida 2, para 1, at 40 weeks' gestation is admitted to the hospital in active labor. The patient reports severe pelvic pain. Pregnancy has been complicated by gestational diabetes. Pregnancy and delivery of her first child were uncomplicated. Current medications include insulin, folic acid, and a multivitamin. Vital signs are within normal limits. The cervix is 100% effaced and 10 cm dilated; the vertex is at -1 station. The fetal heart rate is reactive with no decelerations. Epidural anesthesia is performed and the patient's symptoms improve. Ten minutes later, the patient has dizziness. Her pulse is 68/min, respirations are 16/min, and blood pressure is 90/60 mm Hg. Intravenous fluid resuscitation is begun. Which of the following is the most likely underlying cause of the patient's hypotension? Options: A) Sympathetic block B) Hypovolemia C) Acute pulmonary hypertension D) Aortocaval compression Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.