**Core Concept**
Congenital Adrenal Hyperplasia (CAH) is a group of inherited disorders characterized by the deficiency of enzymes necessary for cortisol production, leading to an overproduction of androgens and potentially life-threatening salt-wasting crises.

**Why the Correct Answer is Right**
The infant's presentation of vomiting, lethargy, dehydration, shock, and hyperpigmentation of the genital skin is classic for a salt-wasting crisis due to CAH. The deficiency of 21-hydroxylase (the most common form of CAH) leads to a decrease in cortisol and aldosterone production, resulting in an inability to retain sodium and water. The hyperkalemia (potassium of 7 meq/l) and hyponatremia (sodium of 124 meq/l) are indicative of aldosterone deficiency. Additionally, the hypoglycemia can be attributed to the lack of cortisol, which is essential for gluconeogenesis. The normal external genitalia and hyperpigmentation of the genital skin suggest a 21-hydroxylase deficiency.

**Why Each Wrong Option is Incorrect**
**Option A:** Adrenal Haemorrhage typically presents with acute abdominal pain and would not explain the infant's electrolyte imbalances.
**Option B:** Acute Gastorenteritis with dehydration would not cause hyperkalemia or hyponatremia.
**Option D:** Hyperaldosteronism would lead to hypertension, hypokalemia, and metabolic alkalosis, which are not present in this case.

**Clinical Pearl / High-Yield Fact**
In infants with CAH, the presence of salt-wasting crisis is an emergency and requires prompt treatment with hydrocortisone and fludrocortisone to replace deficient hormones and correct electrolyte imbalances.

**✓ Correct Answer: A. Congenital Adrenal Hyperplasia (CAH)**