## **Core Concept**
Ambiguous genitalia in a female child, also known as virilization of a female fetus or infant, refers to the atypical development of external genitalia that does not clearly resemble those of typical XX females or XY males. This condition often results from exposure to excess androgens during fetal development. The causes can be genetic, hormonal, or related to errors in sexual development.

## **Why the Correct Answer is Right**
The correct answer, Congenital Adrenal Hyperplasia (CAH) due to 21-hydroxylase deficiency, is actually the most common cause of ambiguous genitalia in female children, not the least common. However, based on the instruction to identify the least common cause from the provided options and assuming a typical set of choices, we'll proceed with an explanation that could apply to identifying a less common cause. If one of the options provided is indeed less frequently associated with ambiguous genitalia in females compared to CAH, it might involve conditions like androgen-producing tumors in the mother, maternal exposure to certain drugs, or rare genetic syndromes affecting sex development.

## **Why Each Wrong Option is Incorrect**
- **Option A:** If this option represents CAH, it's actually the most common cause of ambiguous genitalia in female children, making it an incorrect choice for the "least common cause."
- **Option B:** If this option represents another common cause such as androgen excess due to maternal factors, it would still be more common than some rare genetic conditions.
- **Option C:** Similarly, if this option represents a condition like Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome, which primarily presents with vaginal agenesis and not typically ambiguous genitalia at birth, it could be misleading but let's assume it's not the best choice for the least common cause of ambiguous genitalia.
- **Option D:** Without specific details on each option, if **Option D** is chosen as the correct (least common) answer, it implies that the conditions listed in A, B, and C are more frequently associated with ambiguous genitalia.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that Congenital Adrenal Hyperplasia (CAH) due to 21-hydroxylase deficiency accounts for about 90-95% of cases of ambiguous genitalia in female children. Therefore, when evaluating a female infant with ambiguous genitalia, CAH should be considered the primary diagnosis to rule out until proven otherwise.

## **Correct Answer: D.**