Schematic of the tyrosine degradation pathway. FAH deficiency in hereditary tyrosinemia type I (HT1) leads to accumulation of fumarylacetoacetate and, maleylacetoacetate, with production of succinylacetone (SA) that becomes detectable in the urine and plasma. NTBC (nitisinone) is a potent inhibitor of the 4-HPPD enzyme; (B, C) photomicrograph of liver from an 8-month old patient with HT1 showing typical micro- and macro-nodular cirrhosis (B, magnification, 50×) and a dysplastic nodule (C, magnification, 200×). TAT, tyrosine aminotransferase; 4-HPPD, 4-OH phenylpyruvate dioxygenase; HGD, homogentisate dioxygenase; MAI, maleylacetoacetate isomerase; FAH, fumarylacetoacetate hydrolase.