**Core Concept:**
Periodontal pockets are shallow or deep spaces that form between the teeth and the gums due to periodontal disease. In Russell's periodontal index, the scoring system is used to assess the severity of periodontal disease. Intrabony pockets are specific types of periodontal pockets that involve the attachment of the gum to the tooth root.

**Why the Correct Answer is Right:**
Intrabony pockets are scored as '4' in Russell's periodontal index. This score is attributed to the presence of a deep pocket (>6mm) with attachment loss involving the root surface. The scoring system helps in assessing the extent and severity of periodontal disease, which is crucial in diagnosing and planning treatment.

**Why Each Wrong Option is Incorrect:**
A. A score of '3' is assigned if the pocket is shallow (3-5mm) with attachment loss but does not involve the root surface. This is incorrect as it does not match the definition of an intrabony pocket.
B. A score of '2' is assigned if there is shallow pocket (1-2mm) with attachment loss, but no pocket depth. This is incorrect as it does not involve the attachment loss to the root surface, which is a crucial component of an intrabony pocket.
C. A score of '5' is assigned for a deep pocket (6-8mm) with attachment loss, but no probing depth. This is incorrect, as it does not account for the attachment loss to the root surface, which is a defining characteristic of intrabony pockets.
D. A score of '4' is assigned for shallow pocket (3-5mm) with attachment loss, involving the root surface. This is incorrect as it does not differentiate between shallow pockets with and without attachment loss to the root surface.

**Clinical Pearl:**
In clinical practice, understanding the periodontal pocket depths, probing depths, and attachment loss is essential for diagnosing and managing periodontal diseases. Intrabony pockets are a critical aspect of periodontal disease progression, particularly when assessing treatment outcomes.

**Correct Answer:**
D. A score of '4' is assigned for shallow pocket (3-5mm) with attachment loss, involving the root surface. This is the correct answer because it accounts for both the shallow depth and attachment loss to the root surface, which are key characteristics of an intrabony pocket.