1. Presented by Yembur Ahmad, MD and reviewed by Claire Knezevic, PhD

An adult patient presented to the emergency department with diffuse abdominal pain, and 2 days of emesis and diarrhea. Their history was significant for several prior similar presentations, attributed to cannabinoid hyperemesis syndrome due to daily marijuana use, with bouts of emesis occurring at regular intervals (every 1-2 months). On examination, their abdomen was diffusely tender to palpation. The patient was diagnosed with another episode of cannabinoid hyperemesis syndrome, managed with supportive therapy, and discharged home.
A few months later, the patient presented again with severe abdominal pain. However, this time the patient developed hypertensive emergency and posterior reversible encephalopathy syndrome (PRES), requiring admission to the ICU. During this admission, a random urine porphobilinogen was found to be elevated. A presumptive diagnosis of acute porphyria was made, pending further workup with specific enzyme testing, but thought to be most consistent with acute intermittent porphyria (AIP).

Question: Which of the following enzymes is affected in patients with acute intermittent porphyria (AIP)?