Congenital hypothyroidism (Cretinism) is most commonly caused by which of the following in developed countries?

Correct answer: Thyroid dysgenesis (agenesis or ectopy)

In iodine-sufficient areas, the most common cause of congenital hypothyroidism is thyroid dysgenesis, where the gland fails to develop normally or is in an ectopic location. It is critical to diagnose this early via neonatal screening to prevent intellectual disability.