What is the most likely mechanism of edema in a patient with moderate edema and significant proteinuria?

The most likely mechanism of edema in a patient with moderate edema and significant proteinuria is decreased plasma oncotic pressure. This condition is commonly associated with nephrotic syndrome, where there is significant loss of albumin (a protein) in the urine due to kidney damage.

Albumin plays a crucial role in maintaining plasma oncotic pressure, which is the pressure exerted by proteins in the blood plasma that pulls water into the circulatory system. When a large amount of protein is lost in urine, the serum albumin levels fall, leading to a reduction in oncotic pressure. This decrease allows fluid to leak out of the vascular compartment and into the interstitial space, resulting in edema.

In this scenario, the presence of significant proteinuria coupled with edema strongly suggests that the primary issue is with the loss of protein from the blood, compromising the oncotic pressure and facilitating fluid accumulation in tissues. This is why decreased plasma oncotic pressure is the most plausible explanation for the edema observed in this patient.