**Core Concept**
Rickets is a disease caused by vitamin D deficiency in children, leading to softening and weakening of bones. The earliest signs of rickets are related to the skeletal system and can be identified through radiographic and clinical examinations.

**Why the Correct Answer is Right**
The earliest sign of rickets is bowing of the legs, which occurs due to the softening of the bones. This is because the growth of the bone is not properly regulated, leading to an abnormal curvature of the legs. As the disease progresses, other signs such as craniotabes (softening of the skull bones) and Harrison's groove (indentation of the lower back) may become apparent.

**Why Each Wrong Option is Incorrect**
**Option A:** Metaphyseal fraying is a sign of rickets, but it is not the earliest sign.
**Option B:** Craniotabes is a sign of rickets, but it is not the earliest sign and is more commonly seen in severe cases.
**Option C:** Pelvic ring deformity is a sign of rickets, but it is not the earliest sign and occurs later in the disease process.

**Clinical Pearl / High-Yield Fact**
The earliest sign of rickets is often overlooked due to its subtle presentation, but it is essential to recognize and diagnose the disease early to prevent long-term complications.

**Correct Answer: B. Craniotabes**