**Core Concept**
The patient's presentation of fatigue, malaise, low-grade fever, and ahralgias, combined with elevated BUN and proteinuria, suggests a renal disorder. This scenario is characteristic of an immune-mediated glomerulonephritis, likely due to the presence of autoantibodies.

**Why the Correct Answer is Right**
The patient's symptoms are consistent with Goodpasture's syndrome, a type of rapidly progressive glomerulonephritis (RPGN) caused by the presence of anti-glomerular basement membrane (anti-GBM) antibodies. These autoantibodies target the glomerular basement membrane, leading to inflammation and damage to the kidney. Testing for anti-GBM antibodies will help confirm the diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Testing for antinuclear antibodies (ANA) is more relevant for diagnosing systemic lupus erythematosus (SLE), which can also present with renal involvement, but the presence of ANA is not specific for this diagnosis.

**Option B:** Complement levels, such as C3 and C4, are often decreased in immune-mediated glomerulonephritis, but testing for complement levels is not as specific for Goodpasture's syndrome as testing for anti-GBM antibodies.

**Option C:** Urine culture and sensitivity testing may be necessary to rule out a urinary tract infection, but this would not explain the patient's renal dysfunction and proteinuria.

**Option D:** Serum creatinine levels may be elevated in renal disease, but this is a nonspecific finding and would not help make the diagnosis of Goodpasture's syndrome.

**Clinical Pearl / High-Yield Fact**
Goodpasture's syndrome is a classic example of a type II hypersensitivity reaction, where autoantibodies target and damage the glomerular basement membrane. This is an important concept to remember for diagnosing and managing immune-mediated renal disorders.

**Correct Answer:** C. Testing for anti-glomerular basement membrane (anti-GBM) antibodies.