Ramesh was a 17-month-old boy who was first brought to our attention at 6 months old for an episode of Escherichia coli urinary tract infection. He was treated with antibiotics and recovered uneventfully. His ultrasound of the kidneys was normal at that time. He presented to us again for two more episodes of urinary tract infection at 12 months and 14 months old, respectively, and recovered uneventfully. This time, he was admitted again for a febrile urinary tract infection, and his urine culture grew Proteus mirabilis. What could be the underlying causes of Ramesh's recurrent urinary tract infections?