**Core Concept**
The classification of perineal tears is crucial in obstetrics to assess the severity of injury and guide management. The World Health Organization (WHO) classification system categorizes perineal tears into four degrees based on the extent of tissue damage.

**Why the Correct Answer is Right**
A third-degree perineal tear (III) involves the anal sphincter complex, which includes the external anal sphincter (EAS) and the internal anal sphincter (IAS). This type of injury occurs when the perineal muscles are stretched or torn beyond the level of the anal verge. The EAS and IAS are crucial for maintaining anal continence, and a III perineal tear can lead to fecal incontinence and other complications.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so we cannot evaluate it.
**Option B:** A second-degree perineal tear (II) is limited to the perineal skin and perineal muscles but does not involve the anal sphincter complex.
**Option C:** A fourth-degree perineal tear (IV) is the most severe type, involving the anal sphincter complex and the mucosa of the anal canal.
**Option D:** This option is not provided, so we cannot evaluate it.

**Clinical Pearl / High-Yield Fact**
The WHO classification system for perineal tears is as follows: I - perineal skin only, II - perineal muscle, III - anal sphincter complex, and IV - anal sphincter complex and mucosa of the anal canal.

**Correct Answer:** C. Fourth-degree perineal tear