**Core Concept**
Oliguria in the postoperative period, especially following major abdominal surgery, can be a sign of acute kidney injury (AKI) due to various factors such as hypovolemia, nephrotoxic substances, or ischemia. The urinary output of 25 mL in 6 hours is below the normal range, indicating potential renal impairment.

**Why the Correct Answer is Right**
The most helpful diagnostic test in this scenario would be the serum creatinine level. Creatinine is a waste product produced by muscle metabolism that is primarily excreted by the kidneys. An elevated serum creatinine level indicates decreased renal function, which is consistent with the patient's oliguria. This test helps to assess the glomerular filtration rate (GFR) and diagnose AKI early. The other options are not directly related to diagnosing AKI or oliguria.

**Why Each Wrong Option is Incorrect**
**Option A:** Urine output is a subjective measure and can be influenced by various factors, making it less reliable than a serum creatinine level for diagnosing AKI.

**Option B:** Blood urea nitrogen (BUN) levels can also be elevated in AKI, but BUN is more susceptible to changes in hydration status and is not as specific as serum creatinine for diagnosing renal impairment.

**Option C:** Urine sodium levels can be helpful in differentiating between prerenal and intrinsic AKI, but this test is not as immediately available or as directly indicative of AKI as serum creatinine.

**Option D:** Urine output is not a diagnostic test, and while it can indicate the presence of AKI, it is not a specific or sensitive measure of renal function.

**Clinical Pearl / High-Yield Fact**
In the postoperative period, a 0.3 mg/dL increase in serum creatinine is considered a significant change, indicating potential AKI. This change should prompt further investigation and intervention to prevent progression to more severe renal impairment.

**Correct Answer:** B. Blood urea nitrogen (BUN) levels.