You are answering a USMLE-style medical question. Question: A 47-year-old woman comes to the physician for a follow-up examination. She has noticed trembling of her hands for the past 5 months. These movements are only present when she reaches to pick up an object or answer the phone. She has a history of major depressive disorder but has been off medications for the past 3 years. Her father had Parkinson's disease and died 6 months ago. She drinks a glass of wine daily. She appears anxious. Her temperature is 37°C (98.6°F), pulse is 80/min, and blood pressure is 118/74 mm Hg. A low-frequency tremor is present that worsens as she reaches out for an object. She consistently overshoots her target. There is no tremor present when her arms are held outstretched. There is increased tone in the lower extremities and Babinski's sign is positive bilaterally. Which of the following is the most likely cause of this patient's symptoms? Options: A) Cerebellar demyelination B) Degenerated substantia nigra C) Degenerated caudate nucleus D) Subthalamic nucleus stroke " Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.