The abdominal x-rays show a small elliptical opacity just lateral to the transverse process of the left 4th lumbar vertebra. This is likely to be a ureteric calculus as the ureter runs along the tips of the transverse process of the lumbar vertebrae.
In view of the ongoing abdominal pain and blood tests showing a new onset renal impairment, the patient underwent a CT scan which revealed an obstructed left kidney, due to a 6mm left ureteric calculus.
Diagnosing renal stones using only x-rays is inadvisable as plain radiography has been shown to have a sensitivity of only 60% for renal calculus detection.
Reference: Grainger & Allison’s Diagnostic Radiology, 6th edition.