What is the most likely mechanism causing urinary tract infections in a 3-year-old with a history of constipation?

Urinary stasis due to mechanical compression is the most likely mechanism for urinary tract infections (UTIs) in a 3-year-old with a history of constipation. In children, constipation can lead to an enlarged rectum, which may exert pressure on the bladder. This pressure can inhibit proper bladder function and lead to urinary stasis, where urine remains in the bladder for longer periods. When urine is stagnant, it becomes a suitable environment for bacteria to multiply, increasing the risk of infection.

In this scenario, the combination of constipation and resulting mechanical compression on the urinary tract creates a situation where the normal flow of urine is disrupted, thus promoting the development of a UTI. Addressing the constipation can often help alleviate the pressure and resolve the risk of urinary tract infections in these cases.