## Core Concept
Direct pulp capping in primary teeth is a procedure aimed at preserving the vitality of the pulp when it is accidentally exposed during the removal of decay. The success of this procedure depends on various factors including the health of the pulp, the presence of bacteria, and the material used for capping.

## Why the Correct Answer is Right
The high rate of failure after direct pulp capping in primary teeth is primarily due to **pulpal infection**. When a pulp is exposed, either due to caries or mechanical trauma, bacteria can easily invade the pulp chamber, leading to inflammation and potentially necrosis of the pulp tissue. Primary teeth have larger pulps and relatively thinner dentin, making them more susceptible to bacterial invasion. The presence of bacteria in the exposed pulp significantly reduces the chances of successful pulp capping.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but typically, incorrect options might relate to factors like the type of restorative material used, the technique of capping, or systemic factors. Without specific details, we focus on the principle that pulpal infection is a critical factor.
- **Option B:** Similarly, without specifics, we acknowledge that while factors like the size of the exposure or the use of inappropriate capping materials can influence outcomes, they are not the primary reason for the high failure rate in primary teeth.
- **Option C:** This could potentially relate to anatomical or procedural aspects, but again, these are not the main reasons for failure compared to pulpal infection.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the success of direct pulp capping is highly dependent on the **aseptic conditions** and the **health of the pulp** at the time of the procedure. In primary teeth, due to their anatomy and the often-complex caries process, maintaining asepsis and ensuring minimal bacterial load are critical for success.

## Correct Answer: D. pulpal infection.