All of the following structures forms the boundary of the urogenital triangle, EXCEPT:

The perineum is divided by an arbitrary line joining the ischial tuberosities into urogenital triangle and anal triangle. The urogenital triangle is bounded by pubic rami superiorly, the ischial tuberosities laterally, and the superficial transverse perineal muscle posteriorly. Perineal membrane fuher divides anterior triangle into superficial and deep spaces. The perineal membrane attaches laterally to the ischiopubic rami, medially to the distal third of the urethra and vagina, posteriorly to the perineal body and anteriorly to arcuate ligament of the pubis. Ref: Cunningham F.G., Leveno K.J., Bloom S.L., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 2. Maternal Anatomy. In F.G. Cunningham, K.J. Leveno, S.L. Bloom, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e.