A 45-year-old man presents to the office for evaluation of a blistering skin rash. He was out in the sun with his family at a baseball game several days ago. Later that evening, he developed a severe blistering rash on the forearms, hands, neck, and legs. He has tried over-the-counter topical emollients but to little effect. The patient has had similar rashes in the past that eventually self-resolved. The first of such episodes occurred around 10-years ago. Past medical history is also significant for hyperlipidemia
and chronic hepatitis
C virus infection. He does not consume alcohol
, tobacco, or illicit substances. Vitals are within normal limits. Physical examination reveals small ruptured blisters diffusely across the forearms, hands, neck, and lower legs.
A defect in which of the following steps of the heme biosynthesis pathway is most likely responsible for this patient’s symptoms?