**Core Concept**
A neonate with a history of not passing meconium and an absent anal opening with a dimple suggests an anorectal malformation, specifically an imperforate anus. This condition requires a precise diagnostic approach to determine the level and type of malformation, guiding surgical management.

**Why the Correct Answer is Right**
An invertogram is the investigation of choice in this scenario because it allows for the assessment of the rectal pouch and its relationship with the anal dimple. This radiographic study involves placing the neonate in a prone position with the hips flexed, which helps to invert the rectal pouch and visualize its distance from the anal dimple. This information is crucial for surgical planning, as it helps the surgeon determine the appropriate approach and level of anastomosis.

**Why Each Wrong Option is Incorrect**
**Option A:** X-ray erect posture is not useful in this scenario, as it does not provide sufficient information about the anorectal relationship and may even obscure the anal dimple.
**Option B:** X-ray supine posture is also not ideal, as it does not facilitate the inversion of the rectal pouch and may lead to a false impression of the anorectal relationship.
**Option C:** Gastrograffin study is not the preferred initial investigation, as it may lead to complications such as peritonitis or intestinal perforation in the neonate.

**Clinical Pearl / High-Yield Fact**
In neonates with anorectal malformations, an invertogram is essential for determining the level and type of malformation, guiding surgical management and improving outcomes.

**Correct Answer Line**
✓ Correct Answer: D. Invertogram