**Core Concept**
The classification of perineal tears is based on the extent of tissue involvement, with the anal sphincters being a critical component. The classification helps guide management and predict outcomes.

**Why the Correct Answer is Right**
A perineal tear involving the perineal muscles but sparing the external and internal anal sphincters is classified as a third-degree tear. This type of tear involves the muscles of the perineum, including the bulbocavernosus and superficial transverse perineal muscles, but spares the anal sphincters. The external anal sphincter is responsible for voluntary control over defecation, while the internal anal sphincter is a smooth muscle layer that provides involuntary control. The sparing of these sphincters suggests a more limited tear.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so we'll move on to the other options.

**Option B:** A second-degree tear involves only the perineal skin and mucous membrane, without muscle involvement.

**Option C:** A fourth-degree tear involves the perineal muscles, anal sphincters, and the mucosa of the anal canal.

**Clinical Pearl / High-Yield Fact**
When assessing a perineal tear, it's essential to examine the integrity of the anal sphincters, as this will guide the classification and inform management decisions.

**Correct Answer:** C.