**Core Concept:**
Cap stage is a crucial phase in tooth development, where odontogenic epithelium forms the dental lamina and dental placode. This stage is essential for tooth bud formation and differentiation of dental papilla and dental follicle. Signalling molecules play a vital role in this process, influencing cell differentiation, proliferation, and apoptosis.

**Why the Correct Answer is Right:**
The correct answer, **Option D:** WNT signalling pathway, is responsible for initiating cap stage in tooth development. The WNT pathway is a critical pathway involved in cell differentiation, proliferation, and survival. It plays a key role in tooth development by regulating the expression of genes involved in tooth morphogenesis.

**Why Each Wrong Option is Incorrect:**
Option A (BMP pathway) is involved in tooth morphogenesis, but it is not the primary pathway responsible for initiating the cap stage.
Option B (FGF pathway) is involved in dental papilla formation, not cap stage initiation.
Option C (TGF-beta pathway) is involved in dental follicle formation but not cap stage initiation.

**Clinical Pearl / High-Yield Fact:**
Understanding tooth development and signalling pathways is essential for dental and oral medicine practitioners. Knowledge of these pathways helps in diagnosing and treating developmental defects of teeth, such as cleft lip and palate, ectodermal dysplasias, and tooth agenesis.

**Question:** During cap stage signalling in the odontogenic epithelium is done by:

A. BMP pathway
B. FGF pathway
C. TGF-beta pathway
D. WNT pathway

**Correct Answer:** D. WNT pathway