**Core Concept**
Severe dehydration in an older child is a critical condition characterized by significant fluid and electrolyte imbalances. It occurs when the body loses more fluids than it can replenish, leading to a decrease in blood volume and a subsequent decrease in cardiac output. This condition can be caused by various factors, including excessive vomiting, diarrhea, or excessive sweating.

**Why the Correct Answer is Right**
The correct answer is based on the concept that severe dehydration occurs when the fluid loss exceeds the body's ability to compensate for it. In children, this typically occurs when the fluid loss exceeds 10% of the body weight. The body's response to dehydration involves the activation of the renin-angiotensin-aldosterone system (RAAS), which increases the reabsorption of sodium and water in the kidneys. However, in severe dehydration, this compensatory mechanism is overwhelmed, leading to a significant decrease in blood volume and cardiac output.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because mild dehydration typically occurs when the fluid loss is less than 5% of the body weight. At this stage, the body's compensatory mechanisms are still effective, and the child may not exhibit significant symptoms.

**Option B:** This option is incorrect because moderate dehydration typically occurs when the fluid loss is between 5-10% of the body weight. While the child may exhibit some symptoms, the body's compensatory mechanisms are still partially effective.

**Option C:** This option is incorrect because it is not a specific percentage of fluid loss that defines severe dehydration. Severe dehydration is a clinical diagnosis that requires a comprehensive evaluation of the child's symptoms, physical examination, and laboratory results.

**Clinical Pearl / High-Yield Fact**
A key clinical pearl to remember is that the severity of dehydration is often underestimated in children, especially in older children who may not exhibit the typical signs of dehydration, such as sunken eyes or a decreased level of consciousness. Therefore, it is essential to carefully evaluate the child's symptoms and laboratory results to determine the severity of dehydration.

**Correct Answer:** C.