**Core Concept**
DITRA syndrome is a rare genetic disorder characterized by developmental delay, intellectual disability, thyroid dysfunction, and resistance to thyroid hormone. This condition is caused by mutations in genes involved in thyroid hormone signaling.

**Why the Correct Answer is Right**
The correct answer is related to the thyroid hormone receptor gene, specifically the THRβ gene. Mutations in this gene disrupt the thyroid hormone receptor's ability to bind to thyroid hormone, leading to resistance to thyroid hormone. This results in thyroid hormone dysfunction, characterized by elevated levels of thyroid-stimulating hormone (TSH) and low levels of free thyroid hormones.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because mutations in the TSH receptor gene (TSHR) lead to conditions like congenital hypothyroidism, not DITRA syndrome.
**Option B:** This option is incorrect because mutations in the thyroid hormone transport protein, transthyretin (TTR), lead to conditions like familial amyloid polyneuropathy, not DITRA syndrome.
**Option C:** This option is incorrect because mutations in the thyroid-stimulating hormone (TSH) gene lead to conditions like isolated TSH deficiency, not DITRA syndrome.

**Clinical Pearl / High-Yield Fact**
DITRA syndrome is a rare but important condition to recognize, as it can present with non-specific symptoms like developmental delay and intellectual disability. It is essential to consider this diagnosis in patients with thyroid dysfunction and intellectual disability.

**Correct Answer: C. THRβ gene.**