**Core Concept**
A perineal tear is classified based on its severity and extent of damage to the perineal muscles and anal sphincter. The classification helps in predicting the risk of complications and guiding management.

**Why the Correct Answer is Right**
A third-degree perineal tear involves damage to the anal sphincter but not the anal mucosa. This classification is further subdivided into three types based on the extent of sphincter damage. In this case, the tear is likely to be a 3a tear, where there is less than 50% of the external anal sphincter thickness affected. The anal mucosa remains intact in this type of tear.

**Why Each Wrong Option is Incorrect**
**Option A:** A first-degree perineal tear only involves the perineal skin and does not involve the muscles or anal sphincter.
**Option B:** A fourth-degree perineal tear involves both the anal sphincter and the anal mucosa, which contradicts the given information.
**Option C:** A second-degree perineal tear involves the perineum and the anal sphincter but not the anal mucosa. However, the extent of sphincter damage is not specified, making it a less specific classification.

**Clinical Pearl / High-Yield Fact**
It's essential to accurately classify perineal tears to predict complications and guide management. The classification is based on the extent of damage to the perineal muscles and anal sphincter, and it's crucial to distinguish between different types of tears to provide optimal care.

**Correct Answer:** C.