A 12-week female presents with apathy, weakness, hypotonia, a large tongue, sluggish movement, abdominal bloating, and an umbilical hernia. What is the diagnosis?

The presentation of the 12-week-old female with apathy, weakness, hypotonia, a large tongue, sluggish movement, abdominal bloating, and an umbilical hernia is highly suggestive of congenital hypothyroidism. This condition is characterized by insufficient thyroid hormone production at birth, leading to a variety of symptoms.

In infants, congenital hypothyroidism often manifests with intellectual disability, growth delays, hypotonia (decreased muscle tone), poor feeding, and a large protruding tongue (macroglossia). The sluggishness and overall apathy can be attributed to the decreased metabolic rate due to low thyroid hormone levels. Abdominal bloating and the presence of an umbilical hernia can further suggest this diagnosis, as there is often a correlation with poor muscle tone and impaired gastrointestinal function in affected infants.

Congenital adrenal hyperplasia would typically present with signs related to adrenal gland dysfunction and abnormal genitalia, which are not indicated in this case. Down syndrome is associated with distinct facial features and may present with hypotonia, but it does not typically include the combination of symptoms described here. Prader-Willi syndrome involves different symptoms, such as hyperphagia and obesity, which are not relevant in this very young infant's presentation.