What is the most likely diagnosis for a 2-week-old female presenting with decreased activity and poor feeding, alongside enlarged fontanelles and jaundice?

The most likely diagnosis for the 2-week-old female presenting with decreased activity, poor feeding, enlarged fontanelles, and jaundice is congenital hypothyroidism. This condition arises from an inability of the thyroid gland to produce sufficient thyroid hormones, which are crucial for normal growth and development, particularly in infants.

The symptoms presented align well with congenital hypothyroidism. Decreased activity and poor feeding can be attributed to the lethargic state often observed in infants that lack adequate thyroid hormone levels. Enlarged fontanelles may indicate delayed closure due to metabolic factors or overall growth issues, which can also occur in cases of congenital hypothyroidism. Jaundice is relatively common in newborns, but in this context, it may reflect underlying metabolic disturbances related to the thyroid hormone deficiency.

Other conditions listed do not present with this specific combination of symptoms as clearly as congenital hypothyroidism. While congenital adrenal hyperplasia might lead to issues such as poor feeding or activity, it would more typically present with adrenal crises or ambiguous genitalia. Inborn errors of metabolism could lead to similar symptoms, but usually involve more pronounced metabolic derangement leading to acute presentations, which is less likely with the gradual symptoms here. Congenital heart defects, while important to consider