What condition may present with hyperechoic renal sinus but is not associated with irregular renal contours?

The presence of hyperechoic renal sinus is a key characteristic in imaging that can indicate the presence of certain conditions. Simple renal cysts create a specific ultrasound appearance where the cyst itself is fluid-filled and anechoic (without echo), but surrounding structures such as the renal sinus can appear hyperechoic due to fat and fibrous tissue. Importantly, simple renal cysts maintain regular contours of the kidney, meaning that despite the presence of the cyst, the overall shape of the renal structure does not become irregular.

This differentiates simple renal cysts from other conditions that could potentially affect the renal contour. For instance, renal atrophy would typically lead to a reduction in renal size, altering the contour reflects. Renal vein thrombosis may lead to swelling and irregularities in form due to compromised blood flow, while acute pyelonephritis often causes changes to kidney shape because of inflammation and potential abscess formation. Thus, the hallmark of simple renal cysts is their benign nature and the preservation of the kidney's regular contours, making them the correct answer in the context of this question.