In which scenario can the renal to aorta ratio (RAR) not be used?

The renal to aorta ratio (RAR) is a diagnostic tool used to assess renal artery stenosis by comparing blood flow velocities in the renal artery to that in the aorta. The RAR is particularly useful when the aortic peak systolic velocity (PSV) falls within a specific range that reflects normal physiological conditions for this comparison.

When the aortic PSV is greater than 90 cm/s or less than 40 cm/s, it can introduce complications in the interpretation of the RAR. An elevated aortic PSV may indicate conditions such as aortic stenosis or other vascular abnormalities, leading to inaccurate assessments of renal artery blood flow. Conversely, a low aortic PSV may suggest hemodynamic instability or insufficient perfusion, which again complicate the use of the RAR. Therefore, in situations where the aortic PSV falls outside the 40 cm/s to 90 cm/s range, the RAR cannot be reliably used, as it would not accurately reflect the hemodynamic status of the renal artery in relation to the aorta.

Thus, this understanding clarifies why option B is the appropriate scenario in which the RAR should not be utilized, as it directly affects the validity of the ratio’s application in clinical assessments.