## **Core Concept**
The clinical presentation of failure to thrive, hypertension, metabolic acidosis, and hyperkalemia in an infant suggests a renal or endocrine disorder affecting electrolyte balance and growth. The combination of hypertension and metabolic acidosis with hyperkalemia points towards a condition that affects kidney function, particularly the ability to regulate electrolytes and acid-base balance.

## **Why the Correct Answer is Right**
The correct answer, **C. Congenital Adrenal Hyperplasia (CAH) due to 21-hydroxylase deficiency**, is a condition that can lead to the described symptoms. Specifically, the salt-wasting form of CAH can cause failure to thrive, and the accumulation of mineralocorticoid precursors can lead to hypertension. However, the more common presentation associated with these symptoms is likely due to another condition but considering CAH: it leads to an imbalance in steroid hormone production, which can affect electrolyte balance indirectly. A more direct cause would involve conditions that primarily affect renal function or electrolyte balance.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is not provided, but typically, one would expect a condition such as renal artery stenosis or another cause of renal hypertension.
- **Option B:** Similarly, not provided, but could include other metabolic disorders.
- **Option D:** Not specified, but could involve other endocrine disorders.

## **Clinical Pearl / High-Yield Fact**
A key condition that presents with hypertension, metabolic acidosis, and hyperkalemia in infants is **Pseudohypoaldosteronism Type 1 (PHA1)**, characterized by renal resistance to aldosterone, leading to failure to thrive, hypertension (due to high levels of mineralocorticoids), and electrolyte imbalances. However, the classic presentation of PHA1 is hypokalemia or hyperkalemia with **hypotension**, not hypertension. A more fitting diagnosis given the symptoms (hypertension, metabolic acidosis, hyperkalemia) could be **Liddle's syndrome**, a condition of overactivity of the epithelial sodium channel (ENaC) in the collecting duct, leading to hypertension and hypokalemia (not hyperkalemia), making it less likely. The actual best match given typical questions would likely focus on disorders of mineralocorticoid action or production.

## **Correct Answer:** C.