Final answer:
The best diagnostic test for flank pain with gross hematuria likely due to nephrolithiasis is a computed tomography (CT or CAT scan), the gold-standard imaging modality for detecting kidney stones.
Step-by-step explanation:
Flank pain with gross hematuria is highly suggestive of nephrolithiasis, commonly known as kidney stones. The best diagnostic testing modality to confirm the presence of kidney stones is a computed tomography (CT or CAT scan). While traditional X-rays may sometimes reveal the stones, a CT scan is considered the gold-standard diagnostic tool as it creates detailed images of the kidneys and urinary tract, allowing for a precise diagnosis. This modality can detect even small stones and provide information on stone size, location, and the potential for obstruction.
Additionally, symptoms like 'loin to groin' colicky pain and hematuria - blood in the urine - are common with kidney stones. For those with suspected urinary tract infections or other differential diagnoses, a culture of the urine sample may be necessary.
Treatment may include pain management, and in the case of obstruction, interventions such as lithotripsy, the insertion of a ureteral stent, or surgery may be required. It is crucial to consult a healthcare provider for proper diagnosis and treatment of kidney stones.