Genetic counseling for lysosomal diseases

A 28-year-old woman comes to the physician for genetic counseling prior to conception. For the past year, she has had intermittent episodes of headache, nausea, abdominal pain, and tingling of her fingers. She also complains of dark urine during the episodes. Her mother and maternal uncle have similar symptoms and her father is healthy. Her husband is healthy and there is no history of serious illness in his family. Serum studies show elevated concentrations of porphobilinogen and δ-aminolevulinic acid. What is the probability of this patient having a child with the same disease as her?

You examine an infant in your office. On exam you observe hypotonia, as well as the findings shown in Figures A and B. You order laboratory testing, which demonstrates the findings shown in Figure C. Which of the following is the most likely pathologic mechanism involved?