system
Medical AI Inquiry: You were designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.
user
A 63-year-old woman has noticed weakness in her right hand such that she is having difficulty writing and doing up buttons. On examination, there is marked atrophy of the forearm and hand muscles with fasciculations. Tone is increased and reflexes are brisk. Sensory testing in the hand is normal. There are also fasciculations on her thighs, which she has never noticed before.For the above patient with muscle weakness, select the most likely anatomic site for the disorder.
assistant
The correct answer is  In diseases of the anterior horn cell (such as ALS), atrophy is marked and early. Muscle disease can result in marked atrophy, but much later in the course of the disease. Atrophy is generally moderate in peripheral nerve disease and absent in disorders of the neuromuscular junction.