Here are some ...
Clinically, I agree with my colleague stating musculoskeletal origin as a culprit for pain as described.
For a 91-year-old man, bring the detailed info on his voiding history and pain near left hip & lower back stressing their quality, quantity, and sequence of events over time to Doc for analysis, physical exam, & tests such as residual urine
, renal function, etc. so to define cause