**Core Concept:** Per rectal bleeding in infants can be caused by various factors, including anatomical, physiological, and pathological conditions. Understanding the anatomy of the rectum and the common causes of bleeding can help in diagnosing the underlying issue.

**Why the Correct Answer is Right:** The correct answer is due to the vascular anatomy of the rectum in infants, which includes the presence of the "van Heurck's vessel" (a thin-walled, short, and tortuous vessel located in the rectal mucosa) that can cause small amounts of blood to appear in the stool. This mechanism is also known as "vascular fragility."

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because the rectum is lined with a mucosa rich in blood vessels, including van Heurck's vessel, which can cause per rectal bleeding.

B. This option is incorrect because the presence of blood in the rectum does not necessarily indicate a pathological cause.

C. This option is incorrect because the rectum is lined with a mucosa rich in blood vessels, making it less likely to be the cause of per rectal bleeding.

D. This option is incorrect as the rectum is lined with a mucosa rich in blood vessels, including van Heurck's vessel, which can cause per rectal bleeding.

**Clinical Pearl:** Understanding the anatomy and common causes of per rectal bleeding in infants can help guide clinical decision-making for further evaluation and management. In most cases, the presence of blood in the rectum indicates a need for further examination, including a digital rectal examination, stool for occult blood, and sometimes colonoscopy for further evaluation.

**Correct Answer:** D. Vascular fragility due to the presence of van Heurck's vessel causes per rectal bleeding in infants.