**Core Concept**
Muscle disuse atrophy is a condition where prolonged periods of inactivity lead to a decrease in muscle mass and strength. This is due to the denervation of muscle fibers, which triggers a cascade of cellular changes that ultimately result in muscle wasting.

**Why the Correct Answer is Right**
The patient's prolonged bedridden period following the RTA is the primary cause of muscle disuse atrophy. During this time, the lack of physical activity leads to a decrease in muscle protein synthesis, an increase in muscle protein degradation, and a subsequent loss of muscle mass. This process is mediated by the ubiquitin-proteasome pathway, which is responsible for breaking down muscle proteins.

**Why Each Wrong Option is Incorrect**
* **Option A:** Muscle atrophy due to RTA is not typically associated with refeeding syndrome, a condition that occurs when a starving patient is suddenly given large amounts of nutrients, leading to electrolyte imbalances and potentially life-threatening complications.
* **Option B:** Muscle disuse atrophy is not primarily caused by disuse of specific muscle groups, but rather by the overall lack of physical activity and muscle contraction.
* **Option C:** While muscle wasting can be seen in patients with cancer, it is not the primary mechanism in this case, where the patient has a history of RTA and prolonged bed rest.

**Clinical Pearl / High-Yield Fact**
The National Academy of Sciences recommends that patients who have been bedridden for extended periods engage in regular physical activity, such as range-of-motion exercises and progressive resistance training, to prevent muscle disuse atrophy and maintain muscle mass.

**Correct Answer: D. Prolonged bed rest leading to muscle disuse atrophy.**