A 25-year-old man is concerned that he looks "weak" and "puny" despite the fact that to neutral observers he appears very muscular. When confronted about his belief he believes he is being humored and that people are in fact making fun of his small size behind his back. He has tried a number of strategies to increase muscle mass, including exercising excessively and using anabolic steroids; however, he remains dissatisfied with his appearance. What is the most likely diagnosis?