**Core Concept:** Ambiguous genitalia is a medical condition in female infants where the external genitalia appear indeterminate, lacking distinct male or female characteristics. It is often due to hormonal imbalances or genetic factors.

**Why the Correct Answer is Right:** The correct answer, **D** (Klinefelter syndrome), is a rare cause of ambiguous genitalia in females because it is a genetic condition affecting only males. Klinefelter syndrome is caused by the presence of an extra X chromosome (XXY), which disrupts normal male development. In such cases, the urogenital sinus and Müllerian ducts do not involute, leading to ambiguous genitalia in the affected individual.

**Why Each Wrong Option is Incorrect:**

A. Turner syndrome (XX) is a genetic condition affecting females, not males, and is characterized by the absence of one X chromosome. It does not lead to ambiguous genitalia but rather results in short stature, primary ovarian insufficiency, and growth deficiencies.

B. Diethylstilbestrol (DES) exposure in utero can cause ambiguous genitalia in females due to disruption of normal female development. However, it is not the least common cause, as it is a known environmental factor affecting exposed fetuses.

C. 5-alpha reductase deficiency (5ARD) is a rare genetic condition that results in the reduced conversion of testosterone to dihydrotestosterone. This results in ambiguous genitalia in affected males. Since it is a genetic disorder affecting males, it is not the least common cause of ambiguous genitalia in females.

E. Androgen insensitivity syndrome (46, XX) is a rare genetic disorder affecting females with the XY chromosomes (46, XX). This disorder is characterized by resistance to androgens, leading to ambiguous genitalia in affected females. It is not the least common cause of ambiguous genitalia in females, as it is a genetic disorder affecting females, not males.

**Clinical Pearl:** Klinefelter syndrome is a rare genetic disorder affecting males, making it the least common cause of ambiguous genitalia in females. It is essential to consider this diagnosis in the differential diagnosis when evaluating ambiguous genitalia in infants, as early intervention is crucial to prevent long-term complications and improve the patient's quality of life.