Which diagnostic test involves inserting a catheter into the bladder to measure urine flow?

Uroflowmetry is the correct answer as it specifically refers to the diagnostic test that measures the rate of urine flow from the bladder. This procedure often involves inserting a catheter to assess how quickly urine is expelled, allowing healthcare providers to evaluate bladder function and identify potential issues related to urinary disorders.

This test is particularly useful for diagnosing conditions such as bladder outlet obstruction or detrusor muscle dysfunction, providing quantitative data on flow rates and patterns. In contrast, other options serve different diagnostic purposes: cystoscopy is an endoscopic procedure used to visualize the interior of the bladder, ultrasound typically assesses anatomical structures without directly measuring flow rates, and urodynamics encompasses a broader range of tests, including those measuring pressure and volume in the bladder. However, uroflowmetry is uniquely focused on the measurement of urine flow specifically.