Which condition is characterized by muscle weakness due to muscle tissue being replaced by fat and fibrous cells?

Muscular Dystrophy is characterized by muscle weakness resulting from the degeneration of muscle tissue, where muscle fibers are progressively replaced by fat and fibrous connective tissue. This condition results in a decline in muscle strength and function over time, leading to progressive disability. The underlying pathology involves a genetic mutation that affects proteins necessary for muscle structure and function.

In contrast, Cystic Fibrosis is a genetic disorder that primarily affects the lungs and digestive system but does not directly involve muscle tissue degeneration. Down Syndrome is a chromosomal condition that causes developmental abnormalities and intellectual disability rather than muscle wasting. Autism Spectrum Disorder is a developmental condition affecting communication and behavior and does not relate to muscle strength or tissue composition. Therefore, the distinctive feature of muscle weakness due to replacement of muscle tissue with fat and fibrous cells is specifically associated with Muscular Dystrophy.