A 2 week old child shows difficulty feeding, somnolence, failure to thrive, and constipation. An ale pediatrician orders blood studies that demonstrate low T4 and high TSH. If appropriate therapy is not promptly instituted, which of the following complications would most likely occur?

**Core Concept**
The child's presentation of low T4 and high TSH levels is indicative of congenital hypothyroidism, a condition where the thyroid gland fails to produce sufficient thyroid hormone. This leads to a decrease in metabolic rate, growth, and development.

**Why the Correct Answer is Right**
The high TSH level is a result of the pituitary gland's attempt to stimulate the thyroid gland to produce more thyroid hormone. However, in congenital hypothyroidism, the thyroid gland is unable to respond to TSH, leading to a buildup of TSH in the blood. If left untreated, this condition can lead to severe intellectual disability, growth retardation, and cretinism. The high TSH level also stimulates the development of goiter, which can lead to respiratory problems due to the enlargement of the thyroid gland.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because while respiratory problems can be a complication of untreated congenital hypothyroidism, it is not the most likely complication.

**Option B:**
This option is incorrect because while seizures can be a complication of untreated congenital hypothyroidism, it is not the most likely complication.

**Option C:**
This option is incorrect because while cardiac problems can be a complication of untreated congenital hypothyroidism, it is not the most likely complication.

**Clinical Pearl / High-Yield Fact**
Untreated congenital hypothyroidism can lead to irreversible intellectual disability and growth retardation, making early diagnosis and treatment crucial.

**Correct Answer:** D.

✓ Correct Answer: D. Mental retardation