A 30-year-old woman suffered a serious contusion

of her left lower extremity while working on a local construction site.

Unfortunately, rhabdomyolysis occurs, which is characterized by the breakdown

of skeletal muscle tissue and the release of its contents, including myoglobin,

into the bloodstream. Myoglobin is a risk to the kidneys, especially when

present in high concentrations in the bloodstream. It fills the renal tubules

in the form of protein depositions and can lead to acute kidney injury. On the

second day of hospitalization, the patient experiences a decrease in urine

output. In addition, the serum creatinine level increases significantly.

Elevated serum creatinine levels indicate a decline in renal function because of

the physiological filtration of creatinine as a waste product. It should be

physiologically filtered by the kidneys and excreted from the body in the

urine. Its elevated levels are therefore a marker of impaired renal function.

The picture shows an image of a renal biopsy obtained. Which of the labelled

parts is primarily used to monitor the osmolarity of tubular fluid?