A 49-year-old man presents with acute-onset flank pain and hematuria. What is the likely diagnosis?

The likely diagnosis in this scenario is nephrolithiasis, commonly known as kidney stones. The acute-onset flank pain experienced by the patient is a hallmark symptom of nephrolithiasis and occurs when a stone obstructs the ureter, leading to increased pressure in the renal pelvis and subsequent pain. The presence of hematuria, or blood in the urine, is also frequently associated with kidney stones, as the stones can cause irritation and damage to the urinary tract lining.

In considering the other potential diagnoses: nephritic syndrome typically presents with a combination of hematuria, hypertension, and edema but is generally not associated with acute, severe flank pain; pyelonephritis would more commonly present with fever, chills, and possibly costovertebral angle tenderness, often in the context of a recent urinary tract infection; and a urinary tract infection (UTI) might include flank pain and hematuria but is less likely to cause the acute and severe pain typically associated with nephrolithiasis.

Thus, given the acute nature of the flank pain and the hematuria, nephrolithiasis is the most fitting diagnosis in this clinical scenario.