Final answer:
The most likely diagnosis for a 19-year-old male with sudden onset of left-sided scrotal pain and loss of the cremasteric reflex is acute epididymitis, an inflammation of the epididymis associated with pain, swelling, and potentially a bacterial infection.
Step-by-step explanation:
A 19-year-old male with sudden onset of left-sided scrotal pain and unilateral loss of the cremasteric reflex most likely has acute epididymitis. Acute epididymitis is an inflammation of the epididymis, a structure within the scrotum where sperm mature and are stored. This condition is characterized by rapid onset of symptoms including pain and swelling in the scrotum, redness, and warmth. It is often caused by a bacterial infection and can be associated with urinary tract infections or sexually transmitted diseases. The loss of the cremasteric reflex, which typically causes the testes to elevate upon stroking the inner thigh, further supports this diagnosis, as this reflex can be dampened or absent in cases of acute epididymitis.