What does a urinalysis with blood but no red blood cells indicate?

When a urinalysis shows blood but no red blood cells, this typically suggests the presence of hemoglobin or myoglobin in the urine rather than intact red blood cells. In this context, rhabdomyolysis is the most fitting condition, as it involves the breakdown of muscle tissue, leading to the release of myoglobin into the bloodstream. When myoglobin is filtered by the kidneys, it can appear in the urine, causing the urine to have a brownish or discolored appearance, which may be described as "blood" in the urinalysis without the presence of red blood cells.

Conditions like leukemia and urinary tract infections may indeed cause blood in the urine, but they typically would result in the presence of leukocytes or red blood cells on urinalysis. Kidney stones can also cause hematuria (blood in urine) leading to intact red blood cells being present as they can irritate the urothelium during passage. Thus, the unique combination of factors indicating muscle breakdown and the resulting presence of myoglobin distinguishes rhabdomyolysis as the correct answer in this scenario.