A 2-year-old boy is brought to the physician because he developed blistering sunburn after a 15-miute exposure. His parents say that he has always burned easily. Examination shows photophobia, anemia, and splenomegaly. The urine stains in his diaper are pink to dark brown. The most likely cause of these symptoms is a defect in which of the following biosynthetic pathways?

Question

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The symptoms described, including blistering sunburn, photophobia, anemia, splenomegaly, and dark urine, are indicative of a defect in the porphyrin biosynthetic pathway, which is associated with conditions like porphyria.  The porphyrias constitute a family of disorders resulting from deficiencies in the various enzymes of heme biosynthesis which is also called the porphyrin pathway. The porphyrias lead to excretion of heme biosynthetic by-products that turn the urine red and when deposited in the teeth turn them reddish brown. Accumulation of these by-products in the skin renders it extremely sensitive to sunlight causing ulceration and disfiguring scars. Increased hair growth (hypertrichosis) is also a symptom of the porphryias leading to appearance of fine hairs over the entire face and on the extremities. This latter symptom lends to the description of "werewolf syndrome" in many porphyria patients.

A 2-Year-Old Boy Is Brought to the Physician Because He