In a newborn with clitoral hypertrophy and labial fusion, what is the most likely cause of the symptoms?

The symptoms described—clitoral hypertrophy and labial fusion—indicate the presence of virilization in a female newborn, which typically results from excessive androgen exposure during embryonic development.

The most likely cause of these symptoms is a defect in adrenal steroid biosynthesis, often seen in conditions like congenital adrenal hyperplasia (CAH). In CAH, specifically due to enzyme deficiencies such as 21-hydroxylase deficiency, there is an accumulation of steroid precursors that lead to increased production of adrenal androgens. The elevated levels of androgens result in the masculinization of female external genitalia, manifesting as clitoral hypertrophy and labial fusion.

This hormonal disturbance highlights how the adrenal glands can influence sexual differentiation and development in fetuses, particularly when androgens are produced in excess due to biosynthetic defects. This condition elucidates the pathophysiology of how prenatal exposure to androgens results in the physical presentation observed in the newborn.