## **Core Concept**
The classification of perineal tears is crucial in obstetrics, particularly in the management and prevention of further complications. Perineal tears are graded based on their severity and the structures involved. The grading system helps in determining the appropriate management and predicting potential complications.

## **Why the Correct Answer is Right**
The correct classification of a perineal tear that involves the perineum and the external anal sphincter but has an intact anal mucosa is crucial. According to the classification system:
- **First-degree tears** involve only the perineal skin and/or vaginal mucosa.
- **Second-degree tears** involve perineum involving perineal muscles but not involving the anal sphincter.
- **Third-degree tears** involve the perineum and the anal sphincter complex:
- **3a:** Less than 50% of external anal sphincter (EAS) thickness torn.
- **3b:** More than 50% of EAS thickness torn.
- **3c:** Both EAS and internal anal sphincter (IAS) torn.
- **Fourth-degree tears** involve the perineum, anal sphincter complex (as in third degree), and anorectal mucosa.

Given that the tear involves the perineum, external anal sphincter, but the mucosa is intact, it falls into the **third-degree** category.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option would be incorrect if it suggested a less severe tear (e.g., first or second degree), which does not involve the anal sphincter.
- **Option B:** If this option suggested a more severe tear (e.g., fourth degree), which involves the mucosa, it would be incorrect because the mucosa is intact in this scenario.
- **Option D:** If this option suggested a completely different classification not aligned with the involvement of the external anal sphincter and intact mucosa, it would be incorrect.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that third-degree perineal tears, which involve the anal sphincter, are further subdivided based on the extent of sphincter damage. The management of these tears often requires surgical intervention and may have implications for future bowel function and continence.

## **Correct Answer:** . 3rd degree.