**Core Concept**
Rickets is a disease caused by vitamin D deficiency in children, leading to softening and weakening of bones. The earliest sign of rickets is due to impaired mineralization of osteoid tissue, which is the unmineralized matrix of bone.

**Why the Correct Answer is Right**
The earliest sign of rickets is a widening of the growth plate (physis) at the ends of the bones, which can be detected on an X-ray. This is because the growth plates are highly active areas of bone growth, and the lack of vitamin D leads to impaired mineralization of the newly formed bone matrix. The widening of the growth plate is due to the accumulation of unmineralized osteoid tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because while softening of the bones (osteomalacia) is a feature of rickets, it is not the earliest sign.
**Option B:** This is incorrect because delayed closure of the fontanelles is a feature of rickets, but it is not the earliest sign.
**Option C:** This is incorrect because while bowing of the legs is a feature of rickets, it is not the earliest sign.

**Clinical Pearl / High-Yield Fact**
In children with rickets, the growth plates are affected, leading to a widening of the growth plate. This is an important clinical finding that can be detected on an X-ray.

**Correct Answer: A. Widening of the growth plate.**