You are answering a USMLE-style medical question. Question: A 58-year-old Caucasian female presents to her primary care physician with concerns about several recent health changes. The patient has recently gained weight and can't seem to exercise like she used to. She also notes that her thinking seems slow. Physical examination shows thickened, dry skin. Serum analysis confirms the physician's suspicions. Which of the following findings was most helpful in the diagnosis? Options: A) High T4 and T3 B) Low T4 and T3 C) High TSH D) Hypercholesterolemia Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.