**Core Concept**
Hypothyroidism in children is a condition characterized by a deficiency of thyroid hormones, which are essential for growth and development. The commonest feature of hypothyroidism in children is a result of the impact of thyroid hormone deficiency on the growth and development process.

**Why the Correct Answer is Right**
The correct answer is related to the growth and development pattern observed in children with hypothyroidism. Growth hormone and thyroid hormone interact to regulate growth. Thyroid hormone deficiency impairs the growth hormone response, leading to a decrease in growth velocity. This results in a characteristic growth pattern, which is the commonest feature of hypothyroidism in children.

**Why Each Wrong Option is Incorrect**
**Option A:** This feature is not the commonest feature of hypothyroidism in children. Mental retardation is a potential complication of untreated congenital hypothyroidism but is not the primary feature.
**Option B:** This feature is not specific to hypothyroidism and can be seen in various other conditions affecting growth and development.
**Option C:** This feature is not the commonest feature of hypothyroidism in children. Although some children with hypothyroidism may have short stature, it is not the primary characteristic.

**Clinical Pearl / High-Yield Fact**
Children with untreated hypothyroidism may exhibit a characteristic growth pattern, often referred to as "growth deceleration," which can be a key indicator of the condition.

**Correct Answer: A. Growth deceleration**