FAD-dependent monooxygenase; part of the gene cluster that mediates the biosynthesis of asperthecin, an anthraquinone pigment (PubMed:18978088, PubMed:21866960). Polyketide synthase (PKS) aptA catalyzes the formation of the aromatic polyketide from acetyl coenzyme A and seven malonyl coenzyme A molecules (PubMed:18978088). Polyketide is subsequently hydrolyzed by the action of the hydrolase aptB into endocrocin-9-anthrone (PubMed:18978088). Endocrocin-9-anthrone is then oxidized into endocrocin by the monooxygenase aptC (PubMed:18978088). Endocrocin is likely to decarboxylate spontaneously to form emodin which explains why there is no decarboxylase in the asperthecin biosynthesis cluster (PubMed:18978088). Finally, aptC or another endogenous oxygenase catalyzes additional oxidation steps to form asperthecin (PubMed:18978088). 3,6,8,9-tetrahydroxy-1-oxo-3-(2-oxopropyl)-1,2,3,4-tetrahydroanthracene-2-carboxyl-[ACP] + H(+) + NADPH + O2 = 2,3,6,8,9-pentahydroxy-1-oxo-3-(2-oxopropyl)-1,2,3,4-tetrahydroanthracene-2-carboxyl-[ACP] + H2O + NADP(+) Secondary metabolite biosynthesis. Expression is induced during late sexual development in the dark (PubMed:26773375). Fails to produce asperthecin (PubMed:18978088). Belongs to the paxM FAD-dependent monooxygenase family.