**Core Concept**
The World Health Organization (WHO) classification of periradicular diseases is a standardized system used to categorize and diagnose various conditions affecting the periapical region of the tooth. This classification is essential for dental professionals to accurately diagnose and treat periradicular diseases, which can lead to tooth loss if left untreated. The WHO classification is based on the severity and location of the disease process.

**Why the Correct Answer is Right**
The code KO4.4 in the WHO classification of periradicular diseases represents an inflammatory periapical cyst. This type of cyst is a fluid-filled sac that forms in the periapical region of the tooth as a result of chronic inflammation. The cyst is typically associated with a non-vital tooth, and its formation is a response to the bacterial infection in the pulp. The inflammatory periapical cyst can cause significant pain and swelling in the surrounding tissues if left untreated.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the type of periapical cyst.
**Option B:** This option is incorrect because it is not a recognized code in the WHO classification of periradicular diseases.
**Option C:** This option is incorrect because it represents a different type of periapical lesion.

**Clinical Pearl / High-Yield Fact**
It is essential for dental professionals to accurately diagnose and treat periradicular diseases to prevent tooth loss and alleviate patient discomfort. The WHO classification provides a standardized system for diagnosing and categorizing these conditions, allowing for effective treatment planning.

**Correct Answer: D. Inflammatory periapical cyst.**