Which of the following signs might NOT be indicative of postrenal failure?

The choice indicating "consistent low sodium" as a sign that might not be indicative of postrenal failure is accurate because sodium levels can vary due to several factors, including hydration status, dietary intake, and various renal pathologies. In the context of postrenal failure, which occurs due to obstruction of urine flow, one would typically observe signs related to the accumulation of waste products and fluid retention rather than specific electrolyte disturbances such as consistently low sodium levels.

Fluid excess, oliguria, and elevations in blood urea nitrogen (BUN) and creatinine are common indicators of postrenal failure. Fluid excess occurs as a result of the kidneys' inability to excrete enough fluid, leading to edema and increased blood volume. Oliguria, defined as a decreased urine output, is a hallmark sign of kidney impairment, including postrenal failure caused by urinary obstruction. Lastly, the elevation of BUN and creatinine levels reflects the kidneys' compromised ability to filter waste products from the blood, which is a direct consequence of the obstructive process.

In contrast, sodium levels can fluctuate based on various factors and do not specifically reflect the obstruction seen with postrenal failure. Thus, this makes "consistent low sodium" less characteristic of postrenal failure, validating the choice.