<prompt> Lower 15th of vagina is formed by?Urogenital sinusParamesonephric ductMesonephric ductMullerian duct  <response>: ANSWER: A Urogenital sinusREF: Grays anatomy 39th ed p-1385, Blueprints Obstetrics and Gynecology by Tamara L. Callahan, Aaron B. Caughey? Page 140, Pediatric and adolescent gynecology by Marc R. Laufer, Donald Peter Goldstein Page 337See APPENDIX-5 below for Adult derivatives and vestigial remains of embryonic urogenital structuresRepeat from Primary DNB CET December 2011,June 2011 The sinovaginal bulb originates from urogenital sinus. Distally sinovaginal bulb and proximally overgrowth of Mullerian duct at Mullerian tubercle results in formation of vaginal plate which on canalization forms upper and lower vagina respectively. The first and second portions of Mullerian duct forms the fimbriae and the fallopian tubes while distal segment forms the uterus and upper vagina.Distal most portion of sinovaginal bulb forms the Hymen. APPENDIX - 5Adult Derivatives and Vestigial Remains of Embryonic Urogenital StructuresEmbryonic StructureMaleFemaleGonadal RidgeCortexMedullaTestisSeminiferous tubules Rete testisOvaryOvarian folliclesMedullaRete ovariiGubernaculumGubernaculum testisOvarian ligamentRound ligament of uterusMesonephric tubulesDuctus efferentesParadidymisEpoophoronParoophoronMesonephric ductAppendix of epididymisDuctus epididymidisDuctus deferensUreter, pelvis, calices, and collecting tubulesEjaculatory duct and seminal vesicleAppendix vesiculosa Duct of epoophoron Duct of GartnerUreter, pelvis, calices, and collecting tubulesParamesonephric duct Mullerian ductAppendix of testisHydatid of MorgagniUterine tubeUterusUpper VaginaUrogenital sinusUrinary bladderUrethra except glandular portionProstatic utricleProstate glandBulboureth rod glandsUrinary bladder UrethraLower Vagina from sinovaginal bulbUrethral and paraurethral glandsGreater vestibular glandsVestibule of vaginaMullerian tubercleSeminal colliculusHymenGenital tuberclePenisGians penisCorpora cavernosa penisCorpus spongiosumClitorisGians clitoridisCorpora cavernosa clitoridisBulb of the vestibuleUrogenital foldsVentral aspect of penisLabia minoraLabioscrotal swellingsScrotumLabia majora 