**Question:** The most abundant principal fibre group in periodontal ligament is:

A. Collagen fibres
B. Elastic fibres
C. Sharpey's fibres
D. Fibroblasts

**Core Concept:**
The periodontal ligament is a connective tissue that connects the cementum of the tooth to the alveolar bone. It plays a crucial role in maintaining the stability of the tooth in the bone socket. The principal fibre groups in the periodontal ligament are collagen fibres, elastic fibres, Sharpey's fibres, and fibroblasts.

**Why the Correct Answer is Right:**
The correct answer is **C. Sharpey's fibres**. Sharpey's fibres are named after Sir William Bowman Sharpey, a British pathologist. These fibres are thin, strong, and flexible collagen fibres that are embedded in the periodontal ligament. They are responsible for transmitting tensile forces between the tooth and the alveolar bone, ensuring the stability of the tooth in the socket. The correct answer is not collagen fibres (option A) or elastic fibres (option B), as these fibre types are not specific to the periodontal ligament.

**Why Each Wrong Option is Incorrect:**

A. Collagen fibres (option A) are the most abundant type of fibre in the periodontal ligament, but they are not the most abundant principal fibre group.

B. Elastic fibres (option B) are also present in the periodontal ligament but are not the most abundant principal fibre group.

**Clinical Pearl:**
Sharpey's fibres are essential for understanding the biomechanics of tooth movement and maintaining the stability of teeth in their respective sockets. This knowledge is crucial for dental practitioners to diagnose, plan treatment, and predict the prognosis of various dental procedures involving periodontal ligament function, such as orthodontic treatment.