A 25- year- old man with no medical or psychiatric hx other than anemia travels frequently for his job. He has trouble on long flights b/c he feels the need to move his legs about and unfasten his seat belt to do so. When he is not permitted to get up, especially on "red eye" flights, he feels symmetrical burning and tingling in his legs. He is also very exhausted and notes that his sleep is not restful. He takes no medications except a daily multivitamin. What is the likely diagnosis?

A. Adjustment disorder.
B. Akathisia.
C. Parkinson's disease.
D. Restless legs syndrome.