Case Report: Mixed gonadal dysgenesis with Müllerian remnants mimicking a prostatic utricle in a child with 45,X/46,XY/47,XYY mosaicism.

Sex-chromosome mosaicism can cause discordant gonadal, ductal, and external genital development. Persistent Müllerian derivatives are classically associated with defects in anti-Müllerian hormone (AMH) production or AMH receptor type 2 (AMHR2) signaling in otherwise normally virilized 46,XY individuals, termed persistent Müllerian duct syndrome (PMDS). In complex mosaicism, however, Müllerian persistence may reflect mixed gonadal dysgenesis (MGD). A male-reared child presented with marked short stature, severe proximal hypospadias, ventral chordee, penoscrotal transposition, an empty left hemiscrotum, and a palpable right testis. At 2 years and 2 months, his height was 77.5 cm (<-3 SD). Peripheral-blood fluorescence in situ hybridization showed mos 47,XYY[108]/46,XY[60]/45,X[32]. Exome sequencing identified no explanatory pathogenic variant, including in AMH or AMHR2, whereas copy-number analysis detected a de novo mosaic Yp11.31-q11.223 duplication concordant with the XYY cell line. The AMH level was >18 ng/mL, the inhibin B level was 166 pg/mL, and testosterone level increased from 0.03 to 4.41 ng/mL after human chorionic gonadotropin stimulation. Bladder distension revealed filling or reflux into a presumed prostatic-utricle-like retrovesical cavity. Computed tomography showed a right intrascrotal testis, a retrovesical cystic lesion, and no identifiable left testis. Combined cystoscopic and laparoscopic assessment revised the diagnosis to a Müllerian remnant complex comprising an infantile uterus, a short vagina, and a left streak-like gonadal-region structure with fallopian-tube-like anatomy. The remnant complex and left gonadal-region tissue were excised with preservation of the right vas deferens, and staged hypospadias reconstruction was initiated. Histopathology confirmed fallopian-tube and dysplastic uterovaginal tissues without identifiable gonadal tissue or malignancy. At 2 years and 10 months, second-stage urethroplasty and correction of penoscrotal transposition achieved a straight penis with a glanular meatus. At approximately 7 months after the second-stage procedure, the child had smooth voiding, favorable uroflowmetry, and no urethrocutaneous fistula or urethral stricture. This case is most appropriately interpreted as 45,X/46,XY/47,XYY mosaic MGD rather than typical 47,XYY syndrome or classic AMH-/AMHR2-related PMDS. Detectable postnatal AMH does not exclude asymmetric impairment of fetal Müllerian regression. Combined cystoscopic and laparoscopic assessment clarified the anatomy, delineated resection boundaries, and guided individualized staged reconstruction.