Preureteric vena cava
A preureteric vena cava, also known as a retrocaval ureter, is a rare congenital malformation of the right human ureter, in which the ureter passes behind the inferior vena cava, causing compression possibly leading to hydronephrosis. The prevalence of this condition is approximately 1 per 1,000 persons, with males 2-3 times more likely than females to develop the condition. Symptoms often do not manifest until those with the condition are aged in their 20s or 30s.

References
"preureteric vena cava" (PDF). Retrieved 15 July 2019.
Morganstern, S. L.; Seery, W. H.; Cole, A. T. (1977). "Preureteric vena cava". Urology. 9 (6): 664–666. doi:10.1016/0090-4295(77)90317-x. ISSN 0090-4295. PMID 883067.