A 73-year-old woman presents with increasing weakness, most noticeable in the legs. She has noticed some cramping and weakness in the upper extremities as well. She has more difficulty removing the lids from jars than before. She has noticed some stiffness in the neck but denies back pain or injury. There is no bowel or bladder incontinence. She takes naproxen for osteoarthritis and is on alendronate for osteoporosis. She smokes one pack of cigarettes daily. The general physical examination reveals decreased range of motion in the cervical spine. On neurological examination, the patient has 4/5 strength in the hands with mild atrophy of the interosseous muscles. She also has 4/5 strength in the feet; the weakness is more prominent in the distal musculature. She has difficulty with both heel walking and toe walking. Reflexes are hyperactive in the lower extremities. Sustained clonus is demonstrated at the ankles. What is the best next step in her management?

**Core Concept**
The patient's presentation of progressive weakness, cramping, and stiffness, particularly affecting the distal musculature, suggests a motor neuron disease. The key underlying principle being tested is the differentiation between upper motor neuron (UMN) and lower motor neuron (LMN) lesions, which is crucial in diagnosing and managing neurological disorders.

**Why the Correct Answer is Right**
The patient's symptoms and examination findings, including decreased strength, atrophy of the interosseous muscles, hyperactive reflexes, and sustained clonus, are characteristic of a lower motor neuron lesion. The presence of distal muscle weakness, particularly affecting the hands and feet, and the absence of sensory deficits or upper motor neuron signs (e.g., spasticity, increased tone) support this diagnosis. The patient's smoking history and use of naproxen may exacerbate her condition.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might consider a diagnosis of myasthenia gravis, which presents with muscle weakness that worsens with activity and improves with rest. However, the patient's symptoms and examination findings do not support this diagnosis, as there is no mention of ocular or bulbar involvement, and the weakness is more prominent in the distal musculature.

**Option B:** This option might consider a diagnosis of Parkinson's disease, which presents with bradykinesia, tremors, and rigidity. However, the patient's symptoms and examination findings do not support this diagnosis, as there is no mention of tremors, bradykinesia, or rigidity, and the weakness is more prominent in the distal musculature.

**Option C:** This option might consider a diagnosis of cervical spondylosis, which presents with neck pain, stiffness, and weakness in the upper extremities. However, the patient's symptoms and examination findings do not support this diagnosis, as there is no mention of neck pain or upper motor neuron signs, and the weakness is more prominent in the distal musculature.

**Option D:** This option is the correct answer, as the patient's symptoms and examination findings are most consistent with a diagnosis of amyotrophic lateral sclerosis (ALS). The presence of both upper and lower motor neuron signs, including distal muscle weakness, atrophy, hyperactive reflexes, and sustained clonus, supports this diagnosis. The patient's smoking history and use of naproxen may exacerbate her condition.

**Clinical Pearl / High-Yield Fact**
ALS is a progressive and fatal motor neuron disease that affects both upper and lower motor neurons, leading to muscle weakness, atrophy, and paralysis. Early recognition and diagnosis of ALS are crucial, as there is no cure, and treatment is primarily focused on managing symptoms and improving quality of life.

**Correct Answer:** D. Order an electromyogram (EMG) and nerve conduction studies (NCS) to confirm the diagnosis of ALS and rule out other motor neuron diseases.

✓ Correct Answer: C. Obtain MRI scan of the cervical spine