**Core Concept**
Oliguria refers to the decreased production of urine, typically defined as a urine output of less than 400 mL in 24 hours. This condition can result from various underlying pathophysiological processes, including reduced renal blood flow, acute kidney injury, or fluid overload.

**Why the Correct Answer is Right**
Oliguria often occurs due to decreased renal perfusion, which can be caused by hypovolemia, heart failure, or medications that constrict blood vessels. The kidneys receive approximately 20-25% of the cardiac output, and a decrease in renal blood flow can lead to a reduction in glomerular filtration rate (GFR) and subsequent oliguria. The kidneys also have a unique ability to autoregulate blood flow, but this mechanism can be overwhelmed in cases of severe hypotension or cardiac failure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because anuria refers to the complete cessation of urine production, which is a more severe condition than oliguria.

**Option B:** This option is incorrect because polyuria refers to the excessive production of urine, often seen in conditions such as diabetes mellitus or primary polydipsia.

**Option C:** This option is incorrect because hematuria refers to the presence of blood in the urine, which is a distinct clinical finding from oliguria.

**Clinical Pearl / High-Yield Fact**
It's essential to differentiate oliguria from anuria, as the underlying causes and management strategies may differ. For example, anuria may require more aggressive fluid resuscitation and hemodynamic support.

**Correct Answer: A. Oliguria refers to the decreased production of urine, typically defined as a urine output of less than 400 mL in 24 hours.