**Core Concept**
Severe dehydration and electrolyte imbalance in a pediatric patient requires prompt correction to restore circulatory volume and prevent organ dysfunction.

**Why the Correct Answer is Right**
The child is presenting with symptoms of severe dehydration, including decreased urine output, lethargy, and "doughy" skin. The expected abnormal electrolytes likely include hyponatremia, hypokalemia, and metabolic acidosis. The goal of initial IV bolus is to rapidly restore circulatory volume and serum electrolyte levels. Normal saline (0.9% NaCl) is the best initial choice because it contains the same concentration of sodium as human plasma, which helps to correct hypovolemia and hyponatremia without causing rapid shifts in serum osmolality.

**Why Each Wrong Option is Incorrect**
* **Option A:** Lactated Ringer's solution is a good choice for fluid resuscitation in adults and children, but it may not be the best initial choice in this case due to the presence of metabolic acidosis. The lactate in Lactated Ringer's could potentially worsen the acidosis.
* **Option B:** D5W (5% dextrose in water) is not a good choice for initial fluid resuscitation because it does not contain any electrolytes and may worsen hyponatremia.
* **Option D:** Hypertonic saline is typically used in cases of severe hyponatremia with symptoms of cerebral edema, which is not the primary concern in this case.

**Clinical Pearl / High-Yield Fact**
In pediatric patients, the initial fluid resuscitation should aim to restore circulatory volume and serum electrolyte levels, and normal saline is often the best choice due to its isotonic nature and lack of potential for causing further electrolyte imbalances.

**Correct Answer:** A. Normal saline (0.9% NaCl)