What substance is most likely to cause assay interference in the measurement of urinary metanephrines?

A 23-year-old barmaid presented with headache, sweating and collapse. She had a past medical history of tension headache and unexplained abdominal pain. Her regular medication included amitriptyline 25 mg at night and paracetamol 1 g as required. She was a smoker and regularly drank alcohol.

On examination, her pulse was 120 beats per minute and her blood pressure was 210/128 mmHg.

Investigations:

24-h urinary metanephrine5.4 µmol (<2)

24-h urinary normetanephrine15.2 µmol (<3)

What substance is most likely to cause assay interference in the measurement of urinary metanephrines?
A . alcohol
B . amitriptyline
C . caffeine
D . nicotine
E . paracetamol