porphobilinogen

noun

por·​pho·​bi·​lin·​o·​gen ˌpȯr-fō-bī-ˈlin-ə-jən 

: a dicarboxylic acid C₁₀H₁₄N₂O₄ that is derived from pyrrole, that is found in the urine in acute porphyria, and that on condensation of four molecules yields uroporphyrin and other porphyrins