**Core Concept**
Ultrasound is a crucial diagnostic modality for evaluating testicular swelling, as it provides detailed images of the testicular parenchyma, testicular tunics, and surrounding structures. This non-invasive imaging technique is particularly useful for distinguishing between various causes of testicular swelling, such as hydrocele, epididymitis, or testicular torsion.

**Why the Correct Answer is Right**
Ultrasound is the investigation of choice for testicular swelling due to its ability to accurately assess testicular blood flow and detect any anatomical abnormalities. It can also help differentiate between a non-viable testis and a viable one, which is essential for guiding management decisions. In addition, ultrasound can guide fine-needle aspiration or biopsy if required.

**Why Each Wrong Option is Incorrect**
**Option A:** **Scrotal exploration** is a surgical procedure that may be necessary in cases of testicular torsion or other emergencies, but it is not the first-line investigation for testicular swelling.

**Option B:** **MRI** can provide detailed images of the scrotal contents, but it is not typically used as the initial investigation for testicular swelling due to its higher cost and limited availability compared to ultrasound.

**Option C:** **Intravenous pyelography (IVP)** is a radiographic study that evaluates the urinary tract, but it is not relevant to the investigation of testicular swelling.

**Clinical Pearl / High-Yield Fact**
In cases of testicular swelling, a high index of suspicion for testicular torsion should be maintained, and prompt ultrasound evaluation is essential to avoid delayed diagnosis and potential loss of the testis.

**Correct Answer:** C.