A 15-year-old male presents with progressive muscle weakness and ptosis. The findings suggest which diagnosis?

The presentation of progressive muscle weakness and ptosis in a 15-year-old male is most consistent with myotonic muscular dystrophy. Myotonic dystrophy is a form of muscular dystrophy characterized by the inability to relax muscles following contraction, often accompanied by symptoms such as muscle weakness, myotonia, and ptosis. This form of dystrophy typically presents in adolescence or adulthood and has a range of effects on muscle groups, including those that lead to eyelid drooping (ptosis).

Myotonic dystrophy is a genetic disorder caused by an expansion of a CTG trinucleotide repeat in the DMPK gene, and it can lead to a variety of muscular and systemic symptoms. The low prevalence of ptosis in other muscular dystrophies, particularly at this age, makes myotonic dystrophy a more likely diagnosis than the other listed conditions.

Facioscapulohumeral muscular dystrophy, while it does cause muscle weakness, typically presents with weakness primarily affecting the face, shoulder, and upper arm, which is distinct from the symptoms described. Duchenne muscular dystrophy usually presents in younger children with a more rapid progression and does not typically present with ptosis. Spinal muscular atrophy is a motor neuron disease