Spinal muscular atrophy (SMA) is defined as which of the following?

• A. A metabolic disorder affecting motor neurons.
• B. An autoimmune disease that attacks the neuromuscular junction.
• C. A spinal cord injury resulting in muscle weakness.
• D. A genetic motor neuron disease causing progressive degeneration of anterior horn cells leading to muscle weakness and atrophy.

Answer: (D)

The main idea is that spinal muscular atrophy is a genetic motor neuron disease in which the anterior horn cells of the spinal cord progressively degenerate. These anterior horn cells are the cell bodies of lower motor neurons that directly stimulate skeletal muscles. When they deteriorate, signals to the muscles diminish, causing progressive, symmetric muscle weakness and atrophy, typically starting proximally, with signs like reduced reflexes and low muscle tone. The condition is inherited—usually autosomal recessive—due to mutations in the SMN1 gene leading to deficient SMN protein needed to sustain motor neurons. It is not a metabolic disorder, an autoimmune attack on the neuromuscular junction, or a spinal cord injury.