**Question:** Which of the following is the least common cause of ambiguous genitalia in a female child?

A. Congenital adrenal hyperplasia
B. 46,XY disorder of sex development
C. Pseudohermaphroditism
D. Turner syndrome

**Core Concept:** Ambiguous genitalia in a female child refers to the presence of ambiguous genitalia in a female fetus or newborn, characterized by genitalia that do not clearly indicate male or female sex. This can be caused by various genetic and hormonal abnormalities.

**Why the Correct Answer is Right:** D. Turner syndrome is the least common cause of ambiguous genitalia in a female child. This condition occurs due to the absence of one X chromosome (45,X karyotype), leading to the characteristic features of Turner syndrome, including short stature, webbed neck, low-set ears, and cardiac, renal, and skeletal abnormalities. Although Turner syndrome can lead to ambiguous genitalia, it is less common compared to other causes.

**Why Each Wrong Option is Incorrect:**

A. Congenital adrenal hyperplasia (CAH) is caused by defects in enzymes involved in cortisol synthesis and results in elevated adrenal androgen levels. While CAH can lead to ambiguous genitalia, it is more common than Turner syndrome.

B. 46,XY disorder of sex development (DSD) refers to conditions where an individual has a karyotype with two X chromosomes, but the development of external genitalia is atypical. This option is incorrect because Turner syndrome is less common than 46,XY DSD, which includes disorders like disorders of sex development (DSD) with XY karyotype.

C. Pseudohermaphroditism refers to the presence of both male and female reproductive structures but does not specifically address the cause of ambiguous genitalia. Although Turner syndrome can present as pseudohermaphroditism, it is still less common than the other options.

**Clinical Pearl:** Turner syndrome is a chromosomal disorder that primarily affects females, while the other options involve hormonal imbalances or structural abnormalities leading to ambiguous genitalia. When evaluating a patient with ambiguous genitalia, a thorough history, physical examination, and appropriate investigations (karyotyping) are essential to reach an accurate diagnosis and guide appropriate management.