**Core Concept**
Hypercalciuria refers to an excess of calcium in the urine, which can increase the risk of developing kidney stones. The definition of hypercalciuria can vary based on age and other factors. In children, the normal range for urinary calcium excretion is typically defined based on the child's body size.

**Why the Correct Answer is Right**
The correct answer is based on the definition of hypercalciuria in children, which is often considered to be more than 4 mg/kg/day of urinary calcium excretion. This value accounts for the child's size and helps to standardize the measurement across different age groups and body sizes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it underestimates the normal range for urinary calcium excretion in children.
**Option B:** This option is also incorrect as it does not accurately reflect the definition of hypercalciuria in children.
**Option C:** Similarly, this option is incorrect as it is not the correct threshold for hypercalciuria.

**Why Each Wrong Option is Incorrect (Continued)**
**Option D:** This option is incorrect because the correct definition of hypercalciuria in children is based on a specific threshold of calcium excretion per kilogram of body weight per day.

**Clinical Pearl / High-Yield Fact**
Hypercalciuria can be a significant risk factor for the development of kidney stones in children. Identifying and managing hypercalciuria is crucial to prevent long-term kidney damage.

**Correct Answer:** Correct Answer: D. 4 mg/kg/24 hours.