Okay, let's tackle this question. The patient is a 2-year-old with diarrhea and failure to thrive. The blood work shows low sodium (122 mmol/L) and high potassium (6 mmol/L). So first, I need to figure out the underlying condition causing these symptoms.

Low sodium (hyponatremia) can be due to various causes like fluid overload, loss of sodium, or excess water retention. High potassium (hyperkalemia) is another key point. Diarrhea is a common symptom in conditions affecting the gastrointestinal tract or certain metabolic disorders. Failure to thrive suggests a chronic issue affecting nutrition absorption or metabolism.

Possible conditions to consider: Celiac disease, which can cause malabsorption leading to diarrhea and failure to thrive. However, celiac typically doesn't cause hyperkalemia; in fact, it might lead to hypokalemia due to diarrhea. Another possibility is congenital adrenal hyperplasia (CAH), which can present with hyperkalemia and hyponatremia. CAH is an autosomal recessive disorder where the adrenal glands can't produce enough cortisol and aldosterone. Aldosterone is crucial for sodium retention and potassium excretion. If aldosterone is low, sodium is lost in urine, leading to hyponatremia, and potassium builds up, causing hyperkalemia. Additionally, CAH can cause vomiting and diarrhea, contributing to failure to thrive.

Other options might include renal tubular acidosis, but that usually presents with metabolic acidosis, which isn't mentioned here. Glucocorticoid deficiency in CAH can also lead to hypoglycemia and poor growth. The combination of hyponatremia, hyperkalemia, and failure to thrive in a young child points towards a salt-wasting type of CAH. Let me check the options again. The correct answer would be Congenital Adrenal Hyperplasia (CAH), specifically 21-hydroxylase deficiency, which is the most common form. So the answer is C.

**Core Concept**
This question tests the recognition of **congenital adrenal hyperplasia (CAH)** as a cause of electrolyte imbalance and failure to thrive. CAH, particularly **21-hydroxylase deficiency**, leads to **aldosterone deficiency**, causing **hyponatremia**, **hyperkalemia**, and **metabolic acidosis**, alongside **glucocorticoid deficiency** contributing to poor growth.

**Why the Correct Answer is Right**
In **CAH due to 21-hydroxylase deficiency**, the enzyme defect blocks cortisol and aldosterone synthesis. Aldosterone deficiency impairs renal sodium reabsorption and potassium excretion, leading to **hyponatremia (Na⁺ 122)** and **hyperkalemia (K⁺ 6)**. The glucocorticoid deficiency causes poor growth (failure to thrive) and increased ACTH-stimulated adrenal androgen production, potentially causing virilization. Diarrhea may arise from associated metabolic acidosis or gastrointestinal dysfunction.

**Why Each Wrong Option is Incorrect**
**Option A:** *Celiac disease* causes malabsorption and diarrhea but typically leads to **hypokalemia** (due to fluid loss) and **hypomagnesemia**, not hyperkalemia.
**Option B:**

✓ Correct Answer: B. 21- Hydroxylase deficiency