**Core Concept**
An imperforate anus is a congenital anomaly characterized by the absence or closure of the anal opening. This condition often presents with a history of failure to pass meconium and requires prompt diagnosis and management. The dimple observed in the neonate is a clinical clue indicative of anorectal malformations.

**Why the Correct Answer is Right**
In the case of an imperforate anus, the rectum ends blindly and the rectal pouch is not connected to the anal opening. The investigation of choice is a **pilonidal barium enema**. This diagnostic test involves the instillation of a barium sulfate solution into the rectal pouch to visualize the extent of the rectal anomaly and the presence of any associated fistulas. The dimple observed in the neonate is a clinical indicator of a low-set anus, which is a common type of anorectal malformation.

**Why Each Wrong Option is Incorrect**
**Option A:** Abdominal X-ray is not the investigation of choice for imperforate anus as it may not provide sufficient information about the extent of the rectal anomaly or the presence of associated fistulas.

**Option B:** Ultrasound may be used to evaluate the presence of associated anomalies, such as spinal dysraphism, but it is not the primary investigation for imperforate anus.

**Option C:** MRI may be used to evaluate the extent of the rectal anomaly and associated fistulas, but it is not the first-line investigation for imperforate anus.

**Clinical Pearl / High-Yield Fact**
The presence of a dimple in the neonate should raise suspicion for an anorectal malformation, and a pilonidal barium enema should be performed promptly to evaluate the extent of the rectal anomaly.

**Correct Answer:** C. MRI may be used to evaluate the extent of the rectal anomaly and associated fistulas, but it is not the first-line investigation for imperforate anus.