A 44-year-old female is started on nortriptyline 25 mg at bedtime for fibromyalgia-associated pain. After 10 days at this low dose, she presents with severe dry mouth, constipation, blurred vision, urinary retention, resting tachycardia of 112 bpm, and excessive daytime sedation. A serum nortriptyline level returns at 310 ng/mL (therapeutic range for depression: 50-150 ng/mL). Pharmacogenomic testing reveals she is a CYP2D6 poor metabolizer with two nonfunctional alleles. Which of the following best explains the relationship between her CYP2D6 status and clinical presentation?