**Core Concept**
Duchenne's muscular dystrophy is a progressive muscle disorder caused by a deficiency of dystrophin, a critical protein for muscle function and integrity. This condition primarily affects skeletal muscles, leading to muscle weakness, wasting, and degeneration.

**Why the Correct Answer is Right**
In Duchenne's muscular dystrophy, the disease primarily affects skeletal muscles, with a predilection for muscles that are rich in fast-twitch fibers. The disease usually spares the diaphragm, which is primarily composed of slow-twitch fibers. The diaphragm is responsible for respiratory function and is crucial for maintaining adequate ventilation.

**Why Each Wrong Option is Incorrect**
**Option A:** Gastrocnemius - This muscle is rich in fast-twitch fibers and is commonly affected in Duchenne's muscular dystrophy, leading to significant weakness and wasting.
**Option B:** Vastus medialis - This muscle is also affected in Duchenne's muscular dystrophy, contributing to proximal muscle weakness and gait disturbances.
**Option D:** Infraspinatus - This muscle is another example of a fast-twitch muscle that is commonly affected in Duchenne's muscular dystrophy, leading to weakness and wasting.

**Clinical Pearl / High-Yield Fact**
Duchenne's muscular dystrophy is an X-linked recessive disorder, meaning it primarily affects males who have only one X chromosome. Females can be carriers, but they are usually asymptomatic or mildly affected due to X-chromosome inactivation.

**Correct Answer: D. Infraspinatus**