A14-year-old boy has been brought to the emergency department by his mother in excruciating pain that is radiating from his scrotum to his inguinal area. the boy's heart rate is 122 beats per minute and he has vomited twice before arrival at the hospital. examination reveals that his scrotum is reddened and slightly swollen and the testes are firm to touch and tender, with extensive cremaster muscle contraction noted. what is the boy's most likely diagnosis?