A27-year-old nurse comes to the emergency department because of nervousness, dizziness, palpitations, and excess perspiration for the past 3 hours. she has had similar episodes over the past 6 months. the symptoms improve following ingestion of orange juice or soft drinks. she says that she has had a great deal of stress. she has been drinking two alcoholic beverages daily for the past month; before this time, she seldom drank alcohol. examination shows no abnormalities. her serum glucose concentration is 30 mg/dl. intravenous glucose is administered, and the patient's symptoms improve. which of the following is the most appropriate next step in diagnosis? (a) liver tests(b) measurement of serum proinsulin and insulin antibodies(c) measurement of serum cortisol and acth concentrations(d) measurement of serum growth hormone and plasma somatomedin-c concentrations(e) measurement of serum insulin and c-peptide concentrations