**Question:** Standard test for diagnosis of IMN (IgA nephropathy)

**Core Concept:** IgA nephropathy is a common primary glomerulonephritis with a characteristic mesangial immune complex deposition involving IgA.

**Why the Correct Answer is Right:** The correct answer, renal biopsy, is the gold standard for diagnosing IgA nephropathy. Renal biopsy involves obtaining a small sample of kidney tissue for microscopic examination, allowing the identification of mesangial immune complex deposition involving IgA. This provides definitive evidence of the disease and guides treatment decisions.

**Why Each Wrong Option is Incorrect:**

A. Serum IgA levels: While elevated IgA levels can indicate IgA nephropathy, it is not a diagnostic test as it measures the amount of IgA in the blood.

B. Urinalysis: While it can show hematuria, proteinuria, and casts, it lacks the specificity to confirm IgA nephropathy diagnosis. A renal biopsy is essential to confirm the diagnosis.

C. Serum creatinine, BUN, or glomerular filtration rate: These tests assess kidney function but do not confirm the diagnosis of IgA nephropathy.

D. Ultrasonography: Ultrasound detects kidney abnormalities but lacks the specificity to confirm IgA nephropathy diagnosis. A renal biopsy is necessary for definitive diagnosis.

**Clinical Pearl:** Renal biopsy is crucial in diagnosing IgA nephropathy, as it allows identification of the characteristic mesangial immune complex deposition involving IgA and guides treatment decisions. Serum IgA levels, urinalysis, serum creatinine, BUN, or GFR may indicate kidney involvement but lack the specificity to confirm the diagnosis of IgA nephropathy.

**Correct Answer:** Renal biopsy (Correct Answer: D)