You are answering a USMLE-style medical question. Question: A 6-day-old male newborn is brought to the physician because he has become increasingly irritable and restless over the past 2 days. During this period, he has had 12 bowel movements. He feeds 10 to 12 times a day. He was born at 38 weeks' gestation and weighed 1800 g (3 lb 15 oz); he currently weighs 1700 g (3 lb 12 oz). His mother has Graves' disease and received propylthiouracil during the last trimester of pregnancy. She has a history of intravenous heroin use. His temperature is 36.9°C (98.4°F), pulse is 180/min, and respirations are 50/min. Examination shows mild diaphoresis and a firm 2-cm midline neck swelling. The lungs are clear to auscultation. Which of the following is the most appropriate next step in management? Options: A) Methimazole and propranolol therapy B) Potassium iodide therapy C) Naloxone therapy D) Calcium gluconate therapy Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.