A 50-year-old client with hypertension is being treated with a diuretic. The client reports muscle weakness and fatigue and the nurse's assessment reveals an irregular heart rate. The nurse should assess the client's levels of which electrolyte?
a) Potassium (K+)
b) Sodium (Na+)
c) Calcium (Ca₂+)
d) Magnesium (Mg₂+)