A 28-year-old woman presents with acute abdominal pain, tachycardia, and neuropsychiatric symptoms following exposure to a sulfonamide drug. Urine porphobilinogen (PBG) is markedly elevated. A 35-year-old man with a lifelong history of photosensitive blistering lesions on sun-exposed skin and hyperpigmentation presents with normal abdominal function and no acute neurological symptoms. Urine uroporphyrin is elevated. Which clinical feature best distinguishes acute intermittent porphyria from porphyria cutanea tarda?