Costovertebral angle tenderness should be assessed whenever the nurse suspects:

Choice A reason: Costovertebral angle tenderness is a classic sign of pyelonephritis, indicating kidney infection or inflammation. Percussing this area elicits pain in renal conditions, making it the correct condition to assess for tenderness. Choice B reason: Cholecystitis causes pain in the right upper quadrant, not the costovertebral angle. Kidney-related pyelonephritis is linked to this tenderness, so this is incorrect for the assessment focus. Choice C reason: Gastric ulcers cause epigastric pain, not costovertebral tenderness, which is renal-specific. Pyelonephritis is the condition associated with this sign, so this is incorrect for the suspected condition. Choice D reason: Pancreatitis presents with abdominal pain, not costovertebral angle tenderness, which indicates kidney issues. Pyelonephritis is the relevant condition, so this is incorrect for the assessment.