A previously healthy 6 week old female infant, is found unresponsive in on the bed. In the emergency depament, she is noted to be well developed and well nourished with normal blood pressure and appearance of the genitalia but with increased pigmentation of her skin. Her blood glucose level is 30 mg/dL. What is the most likely diagnosis?

**Core Concept**
The infant's presentation suggests a condition of excessive cortisol production, leading to hypoglycemia and hyperpigmentation due to increased adrenocorticotropic hormone (ACTH) levels.

**Why the Correct Answer is Right**
The infant's symptoms are consistent with congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency, a genetic disorder leading to impaired cortisol production. This results in the accumulation of 17-hydroxyprogesterone, which is converted to androgens and ACTH, causing hyperpigmentation. The lack of cortisol production leads to hypoglycemia.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it does not account for the infant's hyperpigmentation and hypoglycemia. Congenital hypothyroidism primarily presents with growth retardation, macropcephaly, and delayed developmental milestones.
* **Option B:** This option is incorrect as it does not explain the infant's hyperpigmentation. Phenylketonuria (PKU) primarily presents with developmental delay, seizures, and failure to thrive.
* **Option C:** This option is incorrect as it does not account for the infant's hypoglycemia. Hypoglycemia is not a primary feature of congenital hypopituitarism.
* **Option D:** This option is incorrect as it does not explain the infant's hyperpigmentation. Congenital hypopituitarism primarily presents with growth retardation and delayed developmental milestones.

**Clinical Pearl / High-Yield Fact**
Congenital adrenal hyperplasia due to 21-hydroxylase deficiency is the most common cause of congenital adrenal hyperplasia, accounting for approximately 90-95% of cases.

**Correct Answer:** D.

✓ Correct Answer: B. Familial glucocoicoid deficiency