**Core Concept**
The child's presentation of diarrhea, failure to thrive, and hypokalemia with hyponatremia suggests a diagnosis related to excessive loss of potassium and sodium, possibly due to a gastrointestinal cause.

**Why the Correct Answer is Right**
This presentation is most consistent with congenital chloride diarrhea (CD), a rare genetic disorder characterized by the inability to reabsorb chloride ions in the small intestine. As a result, there is an excessive loss of chloride, sodium, and potassium in the stool. The kidneys attempt to compensate for the loss of sodium by reabsorbing more sodium, leading to hyponatremia. The loss of potassium is due to the accompanying loss of potassium in the stool. The child's failure to thrive is a consequence of malabsorption of essential nutrients.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so it cannot be evaluated.
**Option B:** While cystic fibrosis can cause malabsorption and electrolyte imbalances, it typically presents with meconium ileus in newborns and is associated with elevated sweat chloride levels, not hyponatremia and hypokalemia.
**Option C:** Congenital lactase deficiency would primarily cause malabsorption of lactose, leading to diarrhea, but would not result in the specific electrolyte imbalances seen in this case.

**Clinical Pearl / High-Yield Fact**
Congenital chloride diarrhea is a rare cause of diarrhea and electrolyte imbalances in children, often presenting with hypokalemia and hyponatremia, and is usually diagnosed by demonstrating a low chloride level in the stool.

**Correct Answer:** C.