Review Question - QID 107979

A 25-year-old female presents to the emergency department with vague abdominal pain. She says that the pain began with no noticeable triggers, and she is unable to localize the source of the pain. Further history reveals that she has had two previous laproscopic explorations for similar symptoms and is seeing a therapist for symptoms of anxiety and depression. Physical exam is remarkable only for diffuse abdominal pain with no rebound or localizing signs. A urine sample is obtained and is shown in Figure A on the right (the tube on the left contains water for comparison). This urine sample tests positive for aminolevulinic acid (ALA) and porphobilinogen. A deficiency in which of the following enzymes is most likely responsible for this patient's symptoms?