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A 25-year-old Caucasian female is started on 50 mg per day of desipramine. Within a few days, the patient calls the psychiatrist complaining of severe side effects including constipation, dry mouth, visual difficulties, tremor, and agitated feeling. The patient is on no other medication. The blood level of desipramine is 400 ng/ml. Which of the following is the most likely cause of the patient's symptoms?
A. First pass effect
B. Inhibition of P-glycoprotein
C. Active metabolites of desipramine
D. Poor metabolism of cytochrome P450 substrates
E. Noncompliance with recommended dosing instructions
What is D.?