In male anatomy

Anatomy

Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant 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

Navicular fossa of male urethra

The cavernous portion of the urethra is narrow, and of uniform size in the body of the penis, measuring about 6 mm in diameter; it is dilated behind, within the bulb, and again anteriorly within the glans penis, where it forms the fossa navicularis urethræ....

. Its embryologic

Embryology

Embryology is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage...

 origin is contested, but recent evidence suggests it and the navicular fossa derive from infiltrating endoderm

Endoderm

Endoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and mesoderm , with the endoderm as the intermost layer...

al cells of the urethral plate. In young males, the presence of the lacuna magna is associated with painful urination (dysuria

Dysuria

In medicine, specifically urology, dysuria refers to painful urination.Difficult urination is also sometimes described as dysuria.It is one of a constellation of irritative bladder symptoms, which includes urinary frequency and haematuria....

), bloody urine (hematuria

Hematuria

In medicine, hematuria, or haematuria, is the presence of red blood cells in the urine. It may be idiopathic and/or benign, or it can be a sign that there is a kidney stone or a tumor in the urinary tract , ranging from trivial to lethal...

), and bloody spotting of underwear.