Spinal Muscular Atrophy SMN1 Autosomal Recessive

A 4-month-old boy is brought to the pediatric neurology clinic with progressive hypotonia, weak cry, and difficulty feeding. His parents are unaffected and non-consanguineous. Examination reveals absent deep tendon reflexes, tongue fasciculations, and a "frog-leg" posture. The pedigree pattern shown in the diagram at **A** is consistent with autosomal recessive inheritance. Genetic testing confirms homozygous deletion of the SMN1 gene on chromosome 5q13 with only 1 copy of SMN2. Which of the following best describes the pathophysiologic basis of this condition and the expected clinical course?