**Core Concept**
The patient presents with unilateral scrotal pain and tenderness, indicating a possible testicular or scrotal pathology. This condition requires prompt evaluation to rule out testicular torsion, which is a medical emergency.

**Why the Correct Answer is Right**
The initial investigation of choice is scrotal ultrasound, which helps to differentiate between testicular torsion, epididymitis, and other causes of scrotal pain. Ultrasound can assess testicular blood flow and detect any abnormalities in the testicular and epididymal structures. It is a non-invasive and quick test that can guide further management.

**Why Each Wrong Option is Incorrect**
**Option A:** Physical examination alone may not be sufficient to diagnose the underlying cause of scrotal pain, as it may not accurately assess testicular blood flow or detect subtle abnormalities in the scrotal structures.

**Option B:** Abdominal ultrasound may be useful in evaluating the testes, but it is not the primary investigation of choice for scrotal pain, as it may not provide the necessary detail for scrotal structures.

**Option C:** CT scan is not typically used as a first-line investigation for scrotal pain, as it exposes the patient to radiation and may not provide more information than ultrasound.

**Clinical Pearl / High-Yield Fact**
In cases of suspected testicular torsion, prompt evaluation and surgical exploration are crucial, as the longer the delay, the higher the risk of testicular loss.

**Correct Answer: C. CT scan is not typically used as a first-line investigation for scrotal pain, as it exposes the patient to radiation and may not provide more information than ultrasound.