This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
A patient who was previously with good muscle mass now presents with decreased muscle mass. There is a history of RTA which led him to be bedridden for 6 months. This decrease in muscle mass is best explained by 
Select and explain. 
A. Metaplasia, B. Dysplasia, C. Hypertrophy, D. Atrophy
D: Atrophy
Answer: D: Atrophy 
Explanation: None