**Core Concept**
The clinical scenario presented is a case of severe hyperkalemia, which is a potentially life-threatening condition characterized by elevated serum potassium levels. The peaked T waves on ECG are a classic electrocardiographic finding in hyperkalemia. The goal is to rapidly decrease the serum potassium level to prevent cardiac arrest.

**Why the Correct Answer is Right**
The most rapid way to decrease serum potassium levels is through the administration of calcium gluconate or calcium chloride. This works by stabilizing the cardiac cell membranes and preventing the depolarizing effect of potassium on the heart. Calcium also helps to counteract the effects of potassium on the heart by antagonizing the potassium channels. This effect is immediate and can be seen within minutes of administration. The other options, such as insulin, beta-2 agonists, and sodium polystyrene sulfonate, take longer to work and are not as effective in rapidly decreasing serum potassium levels.

**Why Each Wrong Option is Incorrect**
* **Option A:** Insulin and glucose can help to shift potassium into cells, but this effect is delayed and may take up to 30 minutes to become apparent.
* **Option B:** Beta-2 agonists, such as albuterol, can help to shift potassium into cells, but this effect is also delayed and may take up to 30 minutes to become apparent.
* **Option C:** Sodium polystyrene sulfonate can help to remove potassium from the body, but this effect is slow and may take several hours to become apparent.

**Clinical Pearl / High-Yield Fact**
In cases of severe hyperkalemia, always remember the "ABCs" of treatment: Administer calcium gluconate or calcium chloride to stabilize the heart, and then use insulin and glucose or beta-2 agonists to help shift potassium into cells.

**Correct Answer: D. Calcium gluconate.