**Core Concept**
Acute nephritis is a clinical syndrome characterized by inflammation of the renal glomeruli, leading to the triad of hematuria, proteinuria, and hypertension. This condition often results from an immune-mediated response to glomerular injury, which can be caused by various factors such as infections, autoimmune diseases, or medications.

**Why the Correct Answer is Right**
Acute nephritis is a classic presentation of post-streptococcal glomerulonephritis (PSGN), a common cause of acute nephritis in children and young adults. The immune response to streptococcal infection leads to the deposition of immune complexes in the glomeruli, triggering an inflammatory response and damage to the glomerular basement membrane. This results in the characteristic symptoms of hematuria, proteinuria, and hypertension. The kidneys' ability to filter waste and excess fluids is compromised, leading to the accumulation of fluid and electrolytes, which exacerbates the symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** Nephrotic syndrome is a distinct clinical entity characterized by massive proteinuria (>3.5 g/24 hours), hypoalbuminemia, and edema, but it typically does not present with hematuria or hypertension.

**Option C:** Renal cell carcinoma is a type of kidney cancer that can cause hematuria, but it is not typically associated with proteinuria or hypertension as primary symptoms.

**Option D:** Nephrocalcinosis refers to the deposition of calcium salts in the renal tissue, which can lead to kidney damage and dysfunction, but it is not characterized by the triad of hematuria, proteinuria, and hypertension.

**Clinical Pearl / High-Yield Fact**
In patients with suspected acute nephritis, a thorough history and physical examination should be performed to identify potential triggers, such as recent streptococcal infection or exposure to nephrotoxic medications. A urinalysis and renal function tests should also be performed to confirm the diagnosis and monitor disease progression.

**✓ Correct Answer: B. Acute nephritis**