**Core Concept**
The patient's oliguria is being investigated with laboratory tests to determine the cause of decreased urine production. The given values for urine osmolality, sodium, and urea ratio are key indicators of the patient's renal function and fluid status.

**Why the Correct Answer is Right**
The high urine osmolality (620 mosm/kg) indicates that the kidneys are able to concentrate the urine, which is a sign of appropriate renal function. The low urine sodium (12 mmol/L) suggests that the kidneys are able to reabsorb sodium, which is also a sign of normal renal function. The high urine/plasma urea ratio (13:1) indicates that the kidneys are able to reabsorb urea, which is a sign of appropriate renal function. The combination of these three values is characteristic of prerenal azotemia, which occurs when there is decreased blood flow to the kidneys.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a high urine osmolality and low urine sodium would not be expected in acute kidney injury (AKI), which is characterized by a low urine osmolality and high urine sodium.
**Option B:** This option is incorrect because a low urine osmolality and high urine sodium would not be expected in prerenal azotemia, which is characterized by a high urine osmolality and low urine sodium.
**Option C:** This option is incorrect because a high urine osmolality and low urine sodium would not be expected in postrenal azotemia, which is characterized by a low urine osmolality and high urine sodium.

**Clinical Pearl / High-Yield Fact**
The urine osmolality, sodium, and urea ratio are all important indicators of the patient's renal function and fluid status. A high urine osmolality, low urine sodium, and high urine/plasma urea ratio are characteristic of prerenal azotemia, which occurs when there is decreased blood flow to the kidneys.

**Correct Answer: C. Prerenal azotemia**