**Core Concept**
The child in this scenario is suffering from severe dehydration, which is a life-threatening condition that requires prompt and accurate fluid management. The presence of metabolic acidosis, indicated by a low pH and decreased bicarbonate levels, necessitates the use of a specific type of intravenous (I.V.) fluid that can help correct the acid-base imbalance.

**Why the Correct Answer is Right**
The child requires a fluid that is rich in sodium and chloride to correct the severe dehydration, but also needs a buffer to help correct the metabolic acidosis. The correct choice is a fluid that contains bicarbonate, which will help increase the serum bicarbonate levels and correct the acid-base imbalance. The fluid should also contain some potassium to help replace the lost potassium ions.

**Why Each Wrong Option is Incorrect**
* **Option A:** This fluid is hypotonic and contains very little sodium, which is not suitable for severe dehydration. It also lacks bicarbonate, making it a poor choice for correcting metabolic acidosis.
* **Option B:** This fluid is also hypotonic and contains some potassium, but it lacks the necessary bicarbonate to correct the metabolic acidosis. It may even worsen the condition if used alone.
* **Option C:** This fluid contains some sodium and chloride, but it lacks bicarbonate and may even contain some lactate, which could worsen the metabolic acidosis.

**Clinical Pearl / High-Yield Fact**
In cases of severe dehydration with metabolic acidosis, it's essential to use a fluid that contains bicarbonate to help correct the acid-base imbalance. The choice of fluid should be based on the severity of dehydration and the presence of other electrolyte imbalances.

**Correct Answer:**
* **Normal Saline with 4.3% Sodium Bicarbonate (Hartmann's Solution)**: This fluid contains 130 mEq/L of sodium, 109 mEq/L of chloride, and 28 mEq/L of bicarbonate, making it an ideal choice for correcting severe dehydration and metabolic acidosis in this child.