**Core Concept**
Hyperkalemia is a condition characterized by elevated serum potassium levels, which can lead to cardiac arrhythmias and death. The most rapid way to decrease serum potassium levels is through the administration of intravenous calcium gluconate, which helps to stabilize cardiac membranes, and insulin/glucose, which drives potassium into cells.

**Why the Correct Answer is Right**
The administration of calcium gluconate (Option C) is the most rapid way to decrease serum potassium levels. Calcium ions bind to potassium ions on the cardiac cell membrane, stabilizing the membrane and reducing the risk of cardiac arrhythmias. This effect occurs rapidly, within 1-2 minutes. Insulin and glucose (Option D) also help to drive potassium into cells, but this effect takes longer to occur.

**Why Each Wrong Option is Incorrect**
**Option A:** Hemodialysis is an effective way to decrease serum potassium levels, but it is not the most rapid method. It takes at least 30 minutes to establish dialysis and can take several hours to effectively lower potassium levels.
**Option B:** Sodium polystyrene sulfonate (Kayexalate) is an oral medication that can help to lower potassium levels by exchanging sodium for potassium in the gut. However, it has a slow onset of action and is not the most rapid method for acute management of hyperkalemia.

**Clinical Pearl / High-Yield Fact**
In the management of hyperkalemia, it is essential to remember the "4 Cs": calcium to stabilize cardiac membranes, cardioversion to correct arrhythmias, dialysis to remove excess potassium, and cardiac arrest treatment if necessary.

**Correct Answer:** C. Calcium gluconate is administered to rapidly decrease serum potassium levels.