**Core Concept**
The clinical presentation of acute abdominal pain, constipation, and vomiting in a 30-year-old lady is suggestive of acute intestinal obstruction. The investigation of choice should be a rapid, non-invasive, and widely available imaging modality that can confirm the diagnosis and guide further management.

**Why the Correct Answer is Right**
X-ray abdomen erect posture is the investigation of choice in this scenario because it is a quick and simple imaging modality that can rapidly identify intestinal obstruction. The erect posture helps to visualize the air-fluid levels in the bowel, which is a characteristic finding in intestinal obstruction. This is because the air rises to the top of the bowel, creating a distinct line between the air and fluid levels. This radiographic sign is often referred to as the "step-ladder sign" or "air-fluid level."

**Why Each Wrong Option is Incorrect**
**Option A:** Ba enema is a contrast study that can be used to diagnose intestinal obstruction, but it is not the initial investigation of choice because it is more invasive and time-consuming. Ba enema is usually reserved for patients who have a known or suspected rectal or sigmoid colon obstruction.

**Option C:** USG (ultrasound) can be used to diagnose intestinal obstruction, but it is not the most sensitive or specific imaging modality for this condition. USG may show indirect signs of obstruction, such as bowel dilatation or ascites, but it may not provide a definitive diagnosis.

**Option D:** CT scan is a sensitive and specific imaging modality for diagnosing intestinal obstruction, but it is not the first-line investigation in this scenario because it is more expensive and exposes the patient to radiation. CT scan is usually reserved for patients who have a complex or uncertain diagnosis.

**Clinical Pearl / High-Yield Fact**
In cases of suspected intestinal obstruction, the erect chest X-ray is often the first imaging modality used to identify free air under the diaphragm, which is a sign of perforation. This is an important distinction because perforation is a surgical emergency that requires immediate attention.

**✓ Correct Answer: B. X-ray abdomen erect posture**