Cretinism is a condition of severely stunted physical and mental growth owing to untreated congenital deficiency of thyroid hormone (congenital hypothyroidism) usually owing to maternal hypothyroidism.

Hyposecretion of thyroxine in children causes cretinism.

1. Explanation of hyposecretion of thyroxine:
- Thyroxine is a hormone produced by the thyroid gland.
- It plays a crucial role in regulating metabolism and growth.
- Hyposecretion refers to the underproduction or insufficient secretion of thyroxine.

2. Cretinism and its features:
- Cretinism is a condition caused by the hyposecretion of thyroxine in children.
- It leads to stunted growth and impaired mental development.
- Children with cretinism have a characteristic appearance with short stature, a large head, and a protruding tongue.
- They also have intellectual disabilities and delayed motor skills development.

3. Role of thyroxine in growth and development:
- Thyroxine is essential for normal growth and development in children.
- It promotes the growth of bones, muscles, and other tissues.
- It also influences brain development and cognitive function.
- Insufficient levels of thyroxine during childhood can lead to various developmental abnormalities.

4. Impact of hyposecretion of thyroxine on growth:
- Hyposecretion of thyroxine in children causes a condition called dwarfism.
- Dwarfism refers to abnormally short stature due to slowed bone growth.
- The lack of thyroxine affects the growth plates in bones, leading to a shorter height than average.

5. Differentiating cretinism from other conditions:
- Gigantism refers to excessive growth due to the overproduction of growth hormone during childhood.
- Acromegaly is a condition characterized by excessive growth and enlargement of bones and tissues in adults.
- Both gigantism and acromegaly are caused by the overproduction of hormones, not the underproduction of thyroxine.

In conclusion, the correct answer is option 'D' - Cretinism. Hyposecretion of thyroxine in children causes cretinism, which is characterized by stunted growth and impaired mental development. It is important to diagnose and treat cretinism early to prevent long-term complications and support the child's development.