Which of the following is a treatment for hyperkalemia in this context?

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When potassium is too high, the immediate danger is its effect on cardiac cell membranes, which can lead to dangerous rhythm disturbances. The first step is to stabilize the heart’s membranes so the patient doesn’t develop life-threatening arrhythmias while you address the potassium level itself. Giving a calcium salt intravenously accomplishes this by raising the threshold for excitability and reducing the tendency of hyperkalemia to disrupt cardiac conduction. Calcium chloride provides a rapid, direct source of calcium to the heart and is a standard choice for quick membrane stabilization in hyperkalemia.

The other options don’t provide this immediate protective effect on the myocardium. Sodium bicarbonate can help shift potassium into cells, particularly if acidosis is present, but it does not stabilize cardiac membranes. Atropine targets heart rate through the autonomic nervous system, not the electrolyte imbalance. Magnesium sulfate is used for other rhythm problems (like torsades or certain cases of magnesium deficiency) and is not a primary treatment to counteract the effects of high potassium on the heart.

So, the best action in this context is to administer calcium chloride to stabilize the myocardium as you pursue other measures to reduce the potassium level.