Final answer:
Edema in nephrotic syndrome is caused by decreased plasma oncotic pressure due to substantial loss of protein, especially albumin, from the blood into the urine when the glomeruli are damaged.
Step-by-step explanation:
The edema associated with nephrotic syndrome is primarily caused by decreased plasma oncotic pressure. When the kidneys' glomeruli get damaged in nephrotic syndrome, there's a substantial loss of protein (mainly albumin) into the urine. This loss leads to low serum albumin levels, and since albumin plays a crucial role in maintaining plasma oncotic pressure, its deficiency results in fluid leakage from the blood vessels into the surrounding tissues, presenting as edema. Conditions like glomerulonephritis and renal vein thrombosis could exacerbate this condition further.