**Core Concept**
In a female patient with a sliding hernia, the most useful investigation aims to confirm the presence of a hernia, identify the contents of the hernia sac, and assess the risk of incarceration or strangulation. A sliding hernia is a type of hernia where the hernial sac contains a portion of the intestine, which can cause bowel obstruction or compromise blood supply.

**Why the Correct Answer is Right**
The most useful investigation in this scenario is a **barium meal follow-through**. This radiological examination involves swallowing a barium sulfate solution, which coats the small intestine and allows for visualization of the bowel on X-rays. A barium meal follow-through can confirm the presence of a sliding hernia by demonstrating a loop of intestine protruding through a defect in the abdominal wall, and it can also assess the patency of the bowel lumen and the presence of any bowel obstruction.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasonography is not the most useful investigation for a sliding hernia in females, as it may not provide sufficient detail of the bowel contents and can be operator-dependent.
**Option B:** CT scan is not the first-line investigation for a sliding hernia, as it exposes the patient to radiation and is more expensive than other imaging modalities.
**Option C:** Plain abdominal X-ray may show signs of bowel obstruction or free air under the diaphragm, but it is not specific for diagnosing a sliding hernia and may not provide sufficient detail of the hernial sac contents.

**Clinical Pearl / High-Yield Fact**
Remember that a sliding hernia in females is often a **McBurney's point hernia**, which occurs through a defect in the rectus sheath near the umbilicus. This type of hernia is more common in females due to the wider diameter of the pelvic outlet and the higher incidence of pelvic organ prolapse.

**Correct Answer: C. Plain abdominal X-ray**