Which muscular dystrophy is characterized by slow, progressive muscle weakness in boys and is less severe than Duchenne's?

Becker's muscular dystrophy is characterized by a slower, progressive muscle weakness compared to Duchenne's muscular dystrophy. The condition primarily affects boys and is caused by mutations in the dystrophin gene, similar to Duchenne's, but these mutations usually result in a partially functional dystrophin protein. As a consequence, the onset of symptoms is typically later, and the progression of muscle weakness is more gradual. Individuals with Becker's may maintain some muscle strength into adulthood, which contrasts with the rapid decline seen in Duchenne's muscular dystrophy.

The distinction between these two forms of muscular dystrophy underscores the variability in genetic expression and clinical presentation within this group of disorders. The remaining options represent different types of muscular dystrophy with varying characteristics: Duchenne's is known for its severe and rapid decline in muscle function; limb-girdle muscular dystrophy presents differently, often affecting shoulder and hip muscles; and myotonic dystrophy has unique symptoms including muscle stiffness and myotonia, distinguishing it from the dystrophinopathies. Thus, Becker's muscular dystrophy stands out for its less severe and slower progression of muscle weakness, making it the correct answer.