**Core Concept**
Congenital anorectal malformations (ARM) are a group of anomalies that involve the development of the anorectal region, leading to a range of clinical presentations. The primary goal of radiological diagnosis is to identify the location of the rectal pouch and the nature of the fistula, if present.

**Why the Correct Answer is Right**
The correct answer is based on the principle that X-ray can detect the presence of gas within the rectal pouch or the fistula tract, which is a key diagnostic feature of ARM. This is typically achieved through a plain abdominal X-ray or a barium enema, which can demonstrate the location and size of the rectal pouch and the presence of any fistulas. In the case of ARM, the rectal pouch is often located in an abnormal position, such as in the bladder or the vagina, and the fistula tract may connect the rectal pouch to the skin or other organs.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because X-ray is not the primary diagnostic modality for ARM. While X-ray can provide some information, it is often used in conjunction with other imaging modalities such as ultrasound, MRI, or CT scans to obtain a more detailed diagnosis.
* **Option B:** This option is incorrect because it is not specific to the detection of ARM. X-ray can detect a range of conditions, including bowel obstruction, intussusception, and foreign bodies, but it is not the best choice for diagnosing ARM.
* **Option C:** This option is incorrect because it is not a common use of X-ray in the diagnosis of ARM. While X-ray can be used to detect some complications of ARM, such as bowel obstruction or perforation, it is not the primary diagnostic tool for this condition.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that ARM is often associated with other congenital anomalies, such as VACTERL association (Vertebral defects, Anal atresia, Cardiac defects, Tracheo-Esophageal fistula, Renal anomalies, and Limb abnormalities). A high index of suspicion and a thorough evaluation are essential in diagnosing ARM.

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