Murphy's punch sign

Murphy's punch sign is elicited when gently tapping the area of the back overlying the kidney produces pain in people with an infection around the kidney (perinephric abscess). Because the kidney lies directly below this area, known as the costovertebral angle, tapping disturbs the kidney. Because it is surrounded by infected fluid, this jarring motion produces pain. This medical test was first described by the American surgeon John Benjamin Murphy.