What is the most likely cause of severe flank pain and a radiopaque stone found in the left ureter of a 26-year-old man?

The presence of a radiopaque stone in the left ureter, combined with severe flank pain, strongly suggests the diagnosis of a calcium oxalate stone. Calcium oxalate stones are the most common type of renal stones and are typically radiopaque, meaning they can be easily identified on imaging studies such as X-rays.

Severe flank pain, known as renal colic, occurs when a stone obstructs the ureter, leading to increased pressure in the kidney and distension of the ureter. This pain is often acute and can radiate to the groin, correlating well with the symptoms presented by the patient.

In contrast, while struvite stones are also radiopaque, they are more commonly associated with urinary tract infections and can form as a result of urease-producing bacteria. Uric acid stones are usually radiolucent and therefore would not appear on an X-ray as a radiopaque stone. Cystine stones can be radiopaque but are much less common than calcium oxalate stones and are typically associated with a hereditary condition (cystinuria). Given the demographics and presentation, calcium oxalate stones emerge as the most likely culprit.