A young child presents with proximal muscle weakness, waddling gait, and pronounced calf muscles. What is the likely diagnosis?

The presentation of a young child with proximal muscle weakness, a waddling gait, and pronounced calf muscles strongly suggests Duchenne muscular dystrophy (DMD). This condition is characterized by progressive weakness primarily affecting the proximal muscles, which are the muscles closest to the trunk, such as those of the hips and shoulders. As the disease progresses, this weakness leads to a waddling gait, as the child struggles to maintain balance and stability while walking.

The marked enlargement of the calves, also known as pseudohypertrophy, is a classic sign of DMD. This occurs due to the replacement of muscle tissue with fatty and fibrous connective tissue, resulting in calves that appear larger than normal but are not functionally strong. The onset of symptoms typically occurs between the ages of 2 to 5 years, and the condition primarily affects males due to its X-linkage.

In summary, the combination of proximal muscle weakness, waddling gait, and calf muscle hypertrophy is characteristic of Duchenne muscular dystrophy, leading to its appropriate identification as the likely diagnosis in this scenario.