**Core Concept**
Muscle atrophy or wasting can occur due to various factors, including disuse, denervation, or metabolic disorders. In this scenario, the patient's muscle size reduction is likely due to prolonged immobilization leading to decreased muscle protein synthesis.

**Why the Correct Answer is Right**
The correct answer is C. Temporary reduction in actin and myosin protein synthesis. Prolonged immobilization, such as that caused by the cast, leads to a decrease in muscle protein synthesis. This reduction is primarily due to the decrease in the synthesis of actin and myosin proteins, which are crucial for muscle contraction. As a result, the muscle fibers begin to shrink in size, leading to muscle atrophy. This process is reversible with the resumption of normal muscle activity.

**Why Each Wrong Option is Incorrect**
**Option A:** Decrease in the number of individual muscle fibers in the muscle. This option is incorrect because muscle atrophy due to disuse typically does not result in a decrease in the number of muscle fibers. Instead, the existing fibers shrink in size.

**Option B:** Decrease in blood flow to the muscle caused by constriction from the cast. While constriction from the cast may cause some degree of ischemia, it is unlikely to be the primary cause of muscle atrophy in this scenario. Prolonged immobilization has a more significant impact on muscle protein synthesis than temporary ischemia.

**Option D:** Progressive denervation. This option is incorrect because denervation typically leads to muscle atrophy through a different mechanism, involving the loss of motor neurons and subsequent muscle fiber degeneration. In this scenario, the patient's muscle atrophy is likely due to disuse rather than denervation.

**Clinical Pearl / High-Yield Fact**
It is essential to note that muscle atrophy due to disuse is reversible with the resumption of normal muscle activity. This highlights the importance of early mobilization and rehabilitation in patients with prolonged periods of immobilization.

**✓ Correct Answer: C. Temporary reduction in actin and myosin protein synthesis**