In male anatomy, the lacuna magna (also called Guérin's sinus) is the largest of several recesses in the roof of the navicular fossa of the urethra. Its embryologic origin is contested, but recent evidence suggests it and the navicular fossa derive from infiltrating endodermal cells of the urethral plate. In young males, the presence of the lacuna magna is associated with painful urination (dysuria), bloody urine (hematuria), and bloody spotting of underwear.