Microscopic Müllerian Duct Remnants in a 46,XY Individual with Genetically Confirmed 17α-Hydroxylase/17,20-Lyase Deficiency: an Unexpected Histopathological Finding.

17α-hydroxylase/17,20-lyase deficiency (17OHD) is a rare form of congenital adrenal hyperplasia characterized by impaired cortisol and sex steroid synthesis with accumulation of mineralocorticoid precursors. Although hypertension, hypokalemia, and sexual infantilism represent the classical phenotype, atypical presentations are increasingly recognized. We report a 15-year-old phenotypically female adolescent presenting with primary amenorrhea, absent secondary sexual characteristics, and a 46,XY karyotype. Despite biochemical evidence of complete 17α-hydroxylase/17,20-lyase deficiency, no hypertension was documented on repeated office and home blood pressure measurements, and serum potassium concentrations remained within the reference range during serial testing. Molecular analysis identified a homozygous pathogenic variant in CYP17A1 (c.374G>A; p.Arg125Gln). Laparoscopy demonstrated no macroscopic Müllerian structures; however, histopathological examination of the bilateral gonadectomy specimens revealed bilateral microscopic Müllerian duct remnants within the perigonadal fibrous tissue. Serum anti-Müllerian hormone concentration was 12.58 ng/mL, within the expected range for a pubertal 46,XY individual. Targeted sequencing of AMH and AMHR2, together with additional genes relevant to gonadal development and 46,XY disorders of sex development, identified no pathogenic variants, likely pathogenic variants, or variants of uncertain significance. This case expands the clinicopathological spectrum of complete 17α-hydroxylase/17,20-lyase deficiency by demonstrating that microscopic Müllerian duct remnants may persist despite a serum AMH concentration within the expected range, with no molecular explanation identified by targeted genetic analysis.