A 27-year-old woman goes to an emergency room with severe abdominal pain. She had previously experienced similar episodes of pain that lasted several hours to a few days, but this episode is the most severe. She has also been experiencing nausea, vomiting, and constipation. The physician is left with the impression that she is agitated and somewhat confused, and an accurate history is difficult to elucidate. The patient is sent for emergency laparotomy, but no pathology is noted at surgery. Following the unrevealing surgery, an older surgeon comments that he had once seen a similar case that was actually due to porphyria.The porphyrias are biochemical abnormalities in which of the following pathways?