Following several days of 12-hour daily rehearsals of the symphony orchestra for a performance of a Wagnerian opera, the 52-year-old male conductor experienced such excruciating pain in the posterior aspect of his right forearm that he could no longer direct the musicians. When the maestro's forearm was palpated 2 cm distal to, and posteromedial to, the lateral epicondyle, the resulting excruciating pain caused the conductor to weep. Injections of steroids and rest were recommended to ease the pain. Which of the following injuries is most likely?