**Question:** A neonate is brought with history of not having passed meconium on examination there is no anal opening but a dimple. Investigation of choice is:

A. Anal endoscopy
B. Fecal occult blood test
C. Anorectal ultrasound
D. Anorectal manometry

**Correct Answer:** C. Anorectal ultrasound

**Core Concept:** Anorectal ultrasound is essential in evaluating neonates with suspected imperforate anus, as it allows for direct visualization of the anus and surrounding structures.

**Why the Correct Answer is Right:** Imperforate anus is a congenital malformation where the anus is absent or non-functional. In this case, the only clue is a "dimple" instead of an anal opening. Anorectal ultrasound can visualize the anal sphincters, the rectum, and the surrounding structures, providing crucial information to confirm or refute the diagnosis of imperforate anus.

**Why Each Wrong Option is Incorrect:**

A. Anal endoscopy is an invasive procedure that requires general anesthesia and is not suitable for neonates. It does not provide adequate visualization of the surrounding structures and is therefore less reliable than ultrasound.

B. Fecal occult blood test is a non-invasive test used to detect blood in the stool but does not provide direct visualization of the anus and surrounding structures. It is a diagnostic tool for hemorrhoids or other gastrointestinal bleeding but not suitable for evaluating anorectal malformations.

D. Anorectal manometry is a non-invasive test that measures muscle pressures in the anal sphincters but does not provide direct visualization of the anus and surrounding structures. It does not replace the need for an ultrasound.

**Clinical Pearl:** Anorectal ultrasound is a non-invasive, safe, and accurate diagnostic tool for evaluating neonates with suspected anorectal malformations, including imperforate anus. It is essential in establishing a definitive diagnosis and planning the appropriate management.