Question

A $24$-year-old woman is brought to the emergency department by her roommate due to severe, progressive muscle weakness, dizziness, and palpitations over the past $3$ days. She has a history of generalized anxiety disorder. She takes no prescribed medications but admits to using over-the-counter supplements to help with weight loss. She denies any recent vomiting, diarrhea, or illicit drug use. On physical examination, her body mass index is $18.2$ kg/m $^2$. Blood pressure is $98/60$ mm Hg while supine and $82/50$ mm Hg upon standing. Pulse is $105$/min supine and $125$/min standing. Respiratory rate is $12$/min. Her mucous membranes are dry, and skin turgor is decreased. Neurologic examination reveals symmetrical $3/5$ strength in the proximal upper and lower extremities with diminished deep tendon reflexes.

Laboratory results are as follows:
Serum sodium: $135$ mEq/L
Serum potassium: $2.2$ mEq/L
Serum chloride: $84$ mEq/L
Serum bicarbonate: $38$ mEq/L
Blood urea nitrogen: $35$ mg/dL
Serum creatinine: $1.2$ mg/dL

Arterial blood gas (room air):
pH: $7.52$
pCO $_2$: $49$ mm Hg
pO $_2$: $92$ mm Hg

Which of the following is the most appropriate next step in the evaluation of this patient's condition?