A 55-year-old male presents with acute onset of severe testicular pain, nausea, and vomiting. He has had similar episodes before that resolved spontaneously. On examination, his left testicle is tender with a palpable thickened spermatic cord. The cremasteric reflex is present. Urine dipstick shows leucocytes and nitrites. His temperature is 38.2°C. What is the most likely diagnosis?