Okay, let's tackle this question. The patient is a 1-month-old baby with frequent vomiting, failure to thrive, and moderate dehydration. The lab results show low sodium (122 mEq/L) and high potassium (6.1 mEq/L). The options aren't listed, but common differentials for this presentation include pyloric stenosis, congenital adrenal hyperplasia, and maybe renal tubular acidosis.

First, low sodium and high potassium. Hypokalemia is usually expected in vomiting, but here potassium is elevated. That's a red flag. Hypovolemic hyperkalemia is possible if there's renal loss or a problem with potassium regulation.

Hypovolemia and hyperkalemia together make me think of conditions where the kidneys are not excreting potassium properly. Congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency could fit. In CAH, there's a deficiency leading to cortisol and aldosterone deficiency. Aldosterone is crucial for sodium retention and potassium excretion in the distal tubules. Without it, the baby would lose sodium and retain potassium, leading to hyponatremia and hyperkalemia. Additionally, there's metabolic acidosis. The vomiting and dehydration could be secondary to salt-wasting in CAH.

Pyloric stenosis typically presents with projectile vomiting, but it usually causes metabolic alkalosis due to loss of gastric HCl. That would lead to hypokalemia, not hyperkalemia. So that's less likely here. Renal tubular acidosis (RTA) can cause hyperkalemia but usually with normal or high sodium. Also, RTA would have a non-anion gap metabolic acidosis, but the question doesn't mention pH.

Other possibilities like adrenal insufficiency (Addison's disease) might present similarly, but in newborns, CAH is more common. The age of presentation is key—1 month old is typical for CAH. The electrolyte abnormalities fit the picture of aldosterone deficiency. So the most likely diagnosis is Congenital Adrenal Hyperplasia due to 21-hydroxylase deficiency.

**Core Concept**
This case tests the recognition of electrolyte abnormalities in neonates, particularly **hyponatremia and hyperkalemia** due to **aldosterone deficiency**, as seen in **21-hydroxylase deficiency** (a form of congenital adrenal hyperplasia). Aldosterone regulates sodium and potassium balance in distal renal tubules.

**Why the Correct Answer is Right**
21-hydroxylase deficiency impairs cortisol and aldosterone synthesis. Aldosterone deficiency leads to **sodium wasting (hyponatremia)** and **potassium retention (hyperkalemia)** due to unopposed mineralocorticoid action. The baby's dehydration, vomiting, and electrolytes align with **salt-wasting CAH**, which presents in the first months of life. The high potassium is a key differentiator from pyloric stenosis (which causes hypokalemia).

**Why Each Wrong Option is Incorrect**
**Option A:** Pyloric stenosis causes **hypokalemic metabolic alkalosis** from H+ and K+ loss via vomiting, not hyperkalemia.
**Option B:** Ren

✓ Correct Answer: C. 21–hydroxylase deficiency