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Monorchism (also monorchidism) is the state of having only one testicle within the scrotum.
This can be due to:
One testicle not descending into the scrotum during normal
embryonic or fetal development (3–4% of 'normal' live births), also known as undescended testis or cryptorchidism. In this case the testis is within the abdominal cavity, somewhere along the normal route of descent – most commonly, within the inguinal canal. Such a testis has an increased risk of malignancy. One testicle may disappear during development (the so-called
vanishing testis) due to some intrauterine insult. This is thought to be most likely vascular, such as testicular torsion. One testicle may have been surgically removed through
orchiectomy. One testicle may be injured.
Due to testicular cancer
Due to injury
Archibald Douglas, 4th Earl of Douglas, magnate of the Kingdom of Scotland, and Peer of France. Lost in 1403, while fighting at the Battle of Shrewsbury (The previous year he had lost an eye at the Battle of Homildon Hill). 
Francisco Franco, caudillo of Spain. 
Troy Bayliss, world superbike champion in 2001, 2006 and 2008. In 2007 he lost a testicle during a race at Donington Park. 
Brian Foster, American mixed martial artist.  Blaine Light, South African kick boxer. Lost left testicle due to late kick.
Paul Wood, English rugby league player who suffered a ruptured testicle during a match and subsequently had it removed.  Thurgood Marshall, United States Supreme Court Justice who injured a testicle during a fraternity event in university. 
Monorchism in nonhuman animals
Although extremely rare, monorchism has been observed to be characteristic of some animal species, notably in beetles.
An individual having monorchism can be referred to as
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