Six weeks after Fred broke his leg, the cast is removed. As he steps down from the exam table, his leg gives way, and he falls. Propose a logical explanation.

Fred must have suffered muscle atrophy.

Explanation:

Since Fred broke his leg and a cast was put around his leg, his orthopedic must have suggested restricted movement of his leg as a part of treatment ... Due to this immobilization the muscle mass of his leg may have got wasted and his leg became weak. Being too weak the leg may not have supported the body weight and Fred fell.