**Core Concept**
Nephritic syndrome is characterized by inflammation of the glomeruli, leading to damage and impaired renal function. The resulting loss of protein and electrolytes in the urine, along with the accumulation of sodium and water, contributes to the development of edema.

**Why the Correct Answer is Right**
The correct answer is D. Sodium and water retention. This occurs due to the activation of the renin-angiotensin-aldosterone system (RAAS) in response to decreased renal perfusion and glomerular filtration rate (GFR). The increased levels of aldosterone stimulate the kidneys to reabsorb sodium and water, leading to fluid overload and edema. The sodium and water retention also contribute to hypertension, which is a common feature of nephritic syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** An increase in plasma protein concentration would actually decrease the likelihood of edema, as plasma proteins contribute to oncotic pressure.

**Option B:** A decrease in plasma protein concentration can lead to hypoalbuminemia, which is more characteristic of nephrotic syndrome, not nephritic syndrome.

**Option C:** Reduced plasma osmotic pressure is more closely associated with nephrotic syndrome, where large amounts of protein are lost in the urine, leading to decreased oncotic pressure and edema.

**Clinical Pearl / High-Yield Fact**
It's essential to differentiate between nephritic and nephrotic syndrome, as the underlying pathophysiology and clinical manifestations are distinct. Nephritic syndrome typically presents with hematuria, proteinuria, and hypertension, whereas nephrotic syndrome is characterized by massive proteinuria, hypoalbuminemia, and edema.

**✓ Correct Answer: D. Sodium and water retention**