Physical Therapy Assistant Practice Exam

Which condition is characterized by degeneration and scarring of motor neurons, leading to asymmetric distal-to-proximal weakness?

1. Myasthenia gravis

2. Amyotrophic lateral sclerosis

3. Spinal muscular atrophy

4. Guillain-Barre syndrome

The correct answer is: Amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is characterized by the degeneration and scarring of motor neurons, specifically affecting the upper and lower motor neurons. This leads to a progressive weakness that typically starts distally—often in the hands and feet—and advances proximally as the disease progresses. Patients may initially experience fine motor difficulties and muscle cramps or twitching in the extremities, which can be mistaken for other conditions. As the motor neurons continue to deteriorate, weakness and atrophy become more pronounced throughout the body, often resulting in significant disability. The focus on asymmetric weakness is particularly indicative of ALS, as individuals may experience varied severity in different limbs and muscles. This differentiates it from other conditions like myasthenia gravis, which primarily affects neuromuscular transmission rather than directly damaging motor neurons, resulting in different patterns of weakness that can fluctuate and are often symmetrical. Similarly, spinal muscular atrophy involves a genetic basis with specific motor neuron degeneration but typically leads to a more symmetrical pattern of weakness, and Guillain-Barre syndrome commonly presents with ascending weakness and is often preceded by an infectious process. Therefore, ALS is consistent with the condition described, showing a hallmark of asymmetric distal-to-proximal weakness due to the degeneration of motor