Why does acute intermittent porphyria not cause skin photosensitivity?

• A The accumulated precursors ALA and PBG are not photosensitising porphyrins ✓
• B PBG deaminase is not expressed in the skin
• C Hepatic porphyrins are excreted before reaching skin
• D ALA is rapidly converted to non-toxic compounds in skin

Explanation

Photosensitivity requires accumulation of porphyrins, which absorb light at the Soret band (400 nm) and generate reactive oxygen species. In AIP, PBG and ALA accumulate upstream of porphyrin formation; neither is photoactive. PCT, CEP, and EPP accumulate porphyrins and thus cause photosensitivity.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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