A 32-year-old man from rural Maharashtra presents with recurrent episodes of severe abdominal pain, tachycardia, and confusion after taking a barbiturate for insomnia. Serum porphobilinogen deaminase activity is 50% of normal. His sister has the same enzyme deficiency but remains asymptomatic. A 40-year-old woman with a 10-year history of skin fragility, blistering on dorsal hands, and hyperpigmentation has uroporphyrinogen decarboxylase activity reduced to 20% of normal due to a hepatitis C–related acquired defect. Which biochemical finding best distinguishes acute intermittent porphyria from porphyria cutanea tarda?