**Question:** 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

A. Brain
B. Spinal cord
C. Peripheral nerves
D. Muscle

**Correct Answer:** D. Muscle

**Core Concept:** Muscle disorders can manifest with generalized weakness, fasciculations, and muscle atrophy. The clinical presentation here indicates a disorder affecting muscle tissue.

**Why the Correct Answer is Right:** The patient exhibits symptoms of muscle weakness, fasciculations, and atrophy, which are typical signs of a muscle disorder. Furthermore, the presence of fasciculations on her thighs suggests a peripheral nerve involvement, ruling out options A and C. The brisk reflexes and normal sensory testing in the hand indicate that the disorder is not primarily affecting the spinal cord or brain (options A and C). Therefore, the most likely anatomic site for the disorder is muscle (option D).

**Why Each Wrong Option is Incorrect:**

A. Brain: The patient's brisk reflexes and normal sensory testing exclude a central nervous system disorder, which typically presents with altered reflexes and sensory deficits.

B. Spinal cord: Similar to option A, the brisk reflexes and normal sensory testing make spinal cord involvement less likely. Additionally, the presence of fasciculations on her thighs indicates peripheral nerve involvement, making spinal cord involvement even less likely.

C. Peripheral nerves: The presence of fasciculations on her thighs suggests peripheral nerve involvement, ruling out this option.

D. Muscle: As mentioned earlier, the patient's symptoms and examination findings (weakness, fasciculations, and atrophy) are consistent with a muscle disorder.

**Clinical Pearl:** Fasciculations, or involuntary contractions of muscle fibers, are a common feature of muscle disorders and can be helpful in narrowing down the possible etiologies. In this case, the fasciculations on her thighs point towards a peripheral nerve disorder or muscle disorder. Since the brisk reflexes and normal sensory testing rule out a spinal cord disorder, the most likely cause is a muscle disorder (Option D).