**Question:** Motor neuron disease, TRUE is:

A. Spinal muscular atrophy (SMA)
B. Amyotrophic lateral sclerosis (ALS)
C. Progressive muscular dystrophy
D. Myasthenia gravis

**Core Concept:** Motor neuron disease is a group of neurodegenerative disorders affecting motor neurons in the central nervous system (CNS) and/or the peripheral nervous system (PNS). This leads to progressive muscle weakness and atrophy due to the loss of motor neurons.

**Why the Correct Answer is Right:**

Both Spinal Muscular Atrophy (SMA) and Amyotrophic Lateral Sclerosis (ALS) are motor neuron diseases. SMA is a genetic disorder affecting motor neurons in the spinal cord, leading to muscle weakness and wasting. ALS, on the other hand, primarily affects motor neurons in the brain and spinal cord, leading to similar symptoms.

**Why Each Wrong Option is Incorrect:**

A. Progressive muscular dystrophy (PMD) is a group of genetic disorders characterized by muscle weakness and wasting due to degeneration of skeletal muscle fibers, not motor neurons.

D. Myasthenia gravis is an autoimmune disorder affecting neuromuscular junctions, leading to varying degrees of muscle weakness, but it is not a motor neuron disease.

**Clinical Pearl:** Motor neuron diseases often present with symmetrical muscle weakness, starting in the limbs but can also involve other systems like bulbar or respiratory muscles. Early diagnosis and intervention are crucial to improve prognosis and quality of life for patients.

**Correct Answer:** B. Amyotrophic Lateral Sclerosis (ALS) is the TRUE motor neuron disease option in this question.