**Core Concept**
Congenital hypothyroidism is a condition characterized by the absence or deficiency of thyroid hormone production in newborns, leading to growth and developmental abnormalities. The most common cause of congenital hypothyroidism is related to thyroid gland dysmorphogenesis, which affects the production and secretion of thyroid hormones.

**Why the Correct Answer is Right**
The most common cause of congenital hypothyroidism is a thyroid dysgenesis, which includes thyroid agenesis (absence of the thyroid gland), hypoplasia (underdeveloped thyroid gland), and ectopia (abnormal location of the thyroid gland). This condition arises from the failure of the thyroid gland to develop properly during embryogenesis, resulting in the lack of thyroid hormone production. The deficiency in thyroid hormone production leads to growth and developmental problems, including cretinism.

**Why Each Wrong Option is Incorrect**
**Option A:** Thyroid dyshormonogenesis is a less common cause of congenital hypothyroidism, accounting for about 10-15% of cases. It involves genetic mutations affecting thyroid hormone synthesis.
**Option B:** Neonatal hypothyroidism due to maternal iodine deficiency is rare in developed countries, where iodine supplementation is routine.
**Option C:** Transient congenital hypothyroidism is a rare condition where thyroid hormone levels normalize by 6-12 months of age.

**Clinical Pearl / High-Yield Fact**
Thyroid dysgenesis is the most common cause of congenital hypothyroidism, accounting for about 80-85% of cases. Screening programs for congenital hypothyroidism have significantly reduced the incidence of cretinism.

**Correct Answer:** C.