A 40-year-old man has skin that forms blisters upon sun exposure. Which biochemical pathway is likely defective?

In the context of a 40-year-old man who develops skin blisters upon sun exposure, the defect is likely related to the biochemical pathway involved in heme synthesis. This condition could suggest a form of porphyria, particularly one that results in skin manifestations such as photosensitivity, blisters, and increased fragility of the skin after exposure to sunlight.

Porphyrias arise from deficiencies in enzymes that are critical to the heme biosynthesis pathway. As these porphyrins build up due to the enzymatic block, they can cause phototoxicity, leading to skin symptoms including blister formation upon UV exposure. This is why heme synthesis is the correct pathway identified in this scenario.

In contrast, while melanin synthesis is related to the skin's response to UV light and can contribute to skin protection, a defect in this pathway would not typically result in blistering but rather in pigmentation issues. The other pathways, bile acid synthesis and bilirubin degradation, mainly involve liver function and are less likely to be directly implicated in skin blistering due to sunlight.

Therefore, the correct identification of a defect in heme synthesis aligns with the symptoms observed in this patient, reinforcing the understanding of porphyrias and their clinical manifestations.