How does severe early presentation of classic congenital adrenal hyperplasia (CAH) manifest in newborns?

Severe early presentation of classic congenital adrenal hyperplasia (CAH), particularly the most common form caused by 21-hydroxylase deficiency, manifests in newborns primarily through a salt-wasting crisis and dehydration. This occurs due to a lack of cortisol and aldosterone production, leading to adrenal crisis triggered by the inability to maintain sodium retention and blood pressure.

In newborns with classic CAH, the adrenal glands cannot produce adequate aldosterone, which leads to significant loss of sodium in the urine and thus a lower serum sodium concentration. This salt-wasting condition results in dehydration and can precipitate a life-threatening adrenal crisis. Manifestations include poor feeding, vomiting, failure to thrive, lethargy, and hypotension, all of which are critical and require immediate medical attention.

Although ambiguous genitalia may present due to increased androgen exposure in female newborns, the most critical and immediate life-threatening risk associated with severe CAH is the salt-wasting crisis. The other options either describe conditions that do not occur in the neonatal period (like delayed puberty) or result from less severe forms of CAH, leaving salt-wasting and dehydration as the hallmark of severe early presentation.