What is a characteristic feature of spinal muscular atrophy?

A characteristic feature of spinal muscular atrophy (SMA) is proximal muscle weakness. This condition is primarily characterized by the degeneration of motor neurons in the anterior horn of the spinal cord, leading to muscle atrophy and weakness. The weakness tends to affect the proximal muscles more significantly than distal muscles, which means that individuals with SMA often exhibit difficulty with movements such as lifting the arms or holding their head up, while their distal muscles (like those in the hands and feet) may be less affected initially. This proximal weakness in muscle groups that are responsible for posture and mobility is a hallmark of the condition and plays a pivotal role in the clinical assessment and diagnosis of SMA.

The other options do not accurately represent the defining features of SMA. Increased muscle tone is more characteristic of conditions like spasticity rather than SMA, which typically presents with hypotonia (reduced muscle tone). Symmetric distal weakness may be observed in some other neuromuscular disorders, but SMA is noted for proximal muscle involvement primarily. Severe spinal deformities can develop as a consequence of muscle weakness over time, but they are not a direct characteristic feature of the disorder itself and can vary in severity among individuals with SMA.