What causes edema in nephrotic syndrome?
**Core Concept**
Nephrotic syndrome is characterized by excessive loss of protein in the urine, leading to hypoalbuminemia and subsequent fluid shifts. The primary cause of edema in nephrotic syndrome is the decrease in plasma oncotic pressure due to low albumin levels.
**Why the Correct Answer is Right**
The correct answer is related to the concept of plasma oncotic pressure. Albumin is a large protein that helps maintain blood volume by exerting oncotic pressure. When albumin levels decrease in nephrotic syndrome, the oncotic pressure is reduced, causing fluid to shift from the vascular space to the interstitial space, resulting in edema. This process is mediated by the lymphatic system, which is unable to remove the excess fluid quickly enough.
**Why Each Wrong Option is Incorrect**
* **Option A:** Increased sodium reabsorption is not directly related to the pathophysiology of edema in nephrotic syndrome.
* **Option B:** Activation of the renin-angiotensin-aldosterone system (RAAS) may contribute to fluid retention, but it is not the primary cause of edema in nephrotic syndrome.
* **Option C:** While increased sympathetic tone can contribute to fluid retention, it is not the primary cause of edema in nephrotic syndrome.
**Clinical Pearl / High-Yield Fact**
The loss of albumin in nephrotic syndrome is due to the increased permeability of the glomerular basement membrane, allowing large proteins to pass through and into the urine.
**Correct Answer: C.**