The condition of being born with only one kidney. Unilateral renal agenesis occurs approximately once in every 1,000 live births, tends to affect more males than females, and most often results in a missing left kidney. Unilateral renal agenesis is generally not diagnosed in infancy because it tends not to produce symptoms; instead, the lone kidney grows in size to compensate for the deficit. Nevertheless, this condition can manifest itself in some more mundane ways, such as hypertension or mild hearing loss (since the kidneys develop around the same gestational time as the ears).

According to the disturbing but extremely informative medical text The Developing Human: Clinically Oriented Embryology, unilateral renal agenesis occurs "when the metanephric diverticulum fails to develop or with early degeneration of this ureteric primordium." Since my wife was the one that went to medical school, I will leave this explanation as is and not clutter it with my weak interpretation.

Unilateral renal agenesis should be distinguished from a condition known as Horseshoe Kidney, where two distinct kidneys fuse together at either the superior or inferior poles.