Which of the following can be a cause of disuse atrophy?

Disuse atrophy is the reduction in the size of muscle fibers due to a lack of physical activity or exercise. Bed rest is a significant contributor to this condition because it leads to a reduction in the use of muscles. When muscles are not utilized regularly, such as during a prolonged period of inactivity associated with bed rest, the body begins to break down muscle tissue to preserve energy. This atrophy can affect strength, endurance, and overall muscle mass.

Regular exercise, in contrast, promotes muscle growth and maintenance, making it an ineffective cause of disuse atrophy. Proper nutrition and hydration also play roles in maintaining muscle health and do not contribute to muscle wasting. Therefore, prolonged periods of inactivity, such as bed rest, are the primary reason why disuse atrophy occurs, making it the correct answer.