A 12-year-old girl fractures her right radius and ulna in a motor accident. Her arm is placed in a cast extending over the elbow, which nearly eliminates movement of her forearm muscles. Which of the following will occur in her forearm muscles after several weeks in the cast?

**Core Concept**
Muscle disuse atrophy occurs when a muscle is immobilized for an extended period, leading to a decrease in muscle mass and strength. This is due to the lack of mechanical loading and subsequent reduction in muscle protein synthesis.

**Why the Correct Answer is Right**
When a muscle is immobilized, the usual stimulus for muscle protein synthesis is removed, and the muscle undergoes atrophy. This is because the muscle is no longer being used to generate force, and the normal process of muscle growth and repair is disrupted. The forearm muscles will undergo a decrease in cross-sectional area, leading to a reduction in muscle mass and strength.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because muscle atrophy is not caused by a decrease in nerve supply. The nerve supply to the forearm muscles remains intact, and the atrophy is due to the lack of muscle use.

**Option B:** This option is incorrect because muscle atrophy is not caused by a decrease in blood flow. The blood supply to the forearm muscles remains normal, and the atrophy is due to the lack of muscle use.

**Option C:** This option is incorrect because muscle atrophy is not caused by a decrease in bone density. While immobilization can lead to a decrease in bone density, the primary effect is on the muscle, not the bone.

**Clinical Pearl / High-Yield Fact**
Muscle atrophy can be prevented or slowed down by regular exercise and physical activity, even in the presence of immobilization. This can be achieved through passive range-of-motion exercises or electrical muscle stimulation.

**Correct Answer:** A.

✓ Correct Answer: A. Decreased capillarity