Which diagnostic finding is characteristic of porphyria cutanea tarda?

Porphyria cutanea tarda (PCT) is a type of porphyria characterized by photosensitivity and skin manifestations. One of the hallmark features of PCT is the presence of painless blisters that commonly form on sun-exposed areas of the skin, particularly the dorsum of the hands. These blisters arise due to the fragility of the skin resulting from a deficiency in the enzyme uroporphyrinogen decarboxylase, which leads to the accumulation of porphyrins in the skin.

Additionally, the photosensitivity in PCT results from porphyrins being activated by ultraviolet light, leading to damage to the cutaneous tissues and subsequent blister formation. The painless nature of these blisters is significant as it distinguishes PCT from other blistering conditions.

While the other options listed may be relevant to different medical conditions, they do not pertain to the characteristic findings of porphyria cutanea tarda. For instance, elevated serum keratin levels, hypercalcemia, and increased blood ammonia levels could suggest other metabolic or end-organ dysfunctions, but they are not diagnostic of PCT. Thus, the presence of painless blisters on the dorsum of the hands is a definitive clinical finding associated with porphyria