**Core Concept**
Oliguria is a clinical condition characterized by a reduced urine output. In medical practice, it is essential to identify and manage the underlying cause of oliguria to prevent further complications. The kidneys play a crucial role in regulating fluid balance and electrolyte homeostasis.

**Why the Correct Answer is Right**
Oliguria is generally defined as a urine output of less than 400 mL within a 24-hour period. This decrease in urine production can be due to various factors, including hypovolemia, heart failure, nephrotoxicity, or intrinsic renal disease. The kidneys' ability to concentrate urine is impaired in oliguria, leading to a decrease in the concentration of solutes in the urine.

**Why Each Wrong Option is Incorrect**
**Option A:** Less than 100 mL within a 24-hour period. This option is incorrect because it represents a more severe degree of oliguria, often referred to as anuria.
**Option B:** More than 2 liters within a 24-hour period. This option is incorrect because it represents a normal or even increased urine output, which is not indicative of oliguria.
**Option C:** Less than 200 mL within a 24-hour period. This option is incorrect because it is a more conservative estimate of oliguria and does not accurately reflect the generally accepted definition.

**Clinical Pearl / High-Yield Fact**
In patients with oliguria, it is essential to assess the fluid status and electrolyte balance carefully, as these patients are at risk of developing hypovolemia, hyperkalemia, and other electrolyte imbalances.

**Correct Answer: C. Less than 400 mL within a 24-hour period.