A 15-year-old boy presents to the emergency department with sudden-onset severe scrotal pain of 3 hours duration that woke him from sleep, associated with nausea and vomiting. On examination, he has an absent cremasteric reflex and negative Prehn sign. Scrotal Doppler ultrasound is performed. The structure marked **A** in the diagram shows an absent intratesticular arterial waveform on the affected side, with a flat spectral Doppler trace and no pulsatile flow. The contralateral testis (marked **C**) shows normal biphasic arterial waveform. Above the testis, a whirlpool sign is noted on color Doppler. What is the most likely diagnosis and the most appropriate immediate management?