**Core Concept**
The question tests the understanding of contrast-induced compartment syndrome, a rare but serious complication of intravenous contrast administration. This condition occurs due to the extravasation of contrast, leading to increased pressure within a confined space, compromising blood flow and nerve function.

**Why the Correct Answer is Right**
The child's symptoms, including swelling, numbness, and pain, followed by decreased mobility of the hand, suggest compartment syndrome. The presence of a pulse indicates that the arterial circulation is not compromised, but the increased pressure within the confined space (in this case, the forearm) is causing venous congestion and nerve ischemia. The correct answer involves recognizing the need for prompt intervention to prevent permanent damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Administering more contrast would only exacerbate the situation, leading to further extravasation and increased pressure within the compartment.
**Option B:** Applying a tourniquet would further restrict blood flow to the affected area, worsening the compartment syndrome.
**Option C:** Simply monitoring the patient's condition would allow the condition to progress, potentially leading to irreversible nerve damage and limb dysfunction.

**Clinical Pearl / High-Yield Fact**
In cases of suspected contrast-induced compartment syndrome, prompt recognition and intervention are crucial to prevent long-term sequelae. The "6 Ps" of compartment syndrome - Pain, Pallor, Pulselessness, Paraesthesia, Poikilothermia, and Paralysis - are often remembered to aid in early diagnosis.

**Correct Answer:** D. Fasciotomy should be performed to relieve the increased pressure within the compartment and restore blood flow to the affected area.