Testicular Torsion Absent Doppler

A 24-year-old male presents to the emergency department with acute onset of severe left-sided scrotal pain that awoke him from sleep 3 hours prior. The pain is constant, radiates to the lower abdomen, and is associated with nausea and vomiting. On examination, he has a high-riding affected testis in horizontal lie with absent ipsilateral cremasteric reflex and negative Prehn sign. Urgent scrotal Doppler ultrasound is performed. The structure marked **A** in the diagram shows a flat-line isoelectric waveform with complete absence of arterial and venous Doppler signals from the affected testis, while the contralateral side demonstrates normal low-resistance arterial flow with forward diastolic flow. What is the most likely diagnosis?