Fill in the Blank: Key Electrolyte in Dehydration

A patient in the hospital is diagnosed with dehydration due to inadequate fluid intake. The nurse notes that the patient's urine output has significantly decreased. In assessing the patient's condition, it is essential to monitor their serum electrolyte levels. One of the key electrolytes to assess in the context of fluid imbalances is __________. This electrolyte plays a vital role in neuromuscular function and is critical for maintaining proper fluid balance.