**Core Concept**
Anuria is a medical condition characterized by the reduced production of urine, typically defined by a specific volume threshold below which the kidneys are unable to produce sufficient urine. This condition often indicates severe kidney dysfunction or acute kidney injury (AKI).

**Why the Correct Answer is Right**
The correct answer is 100ml because anuria is generally defined as the production of less than 100ml of urine per day. This low urine output is a hallmark of severe kidney impairment, which can be caused by various factors, including obstruction of the urinary tract, severe dehydration, or the use of nephrotoxic medications. The kidneys' ability to concentrate or dilute urine is compromised, leading to a significant decrease in urine production.

**Why Each Wrong Option is Incorrect**
**Option A:** 400ml is considered oliguria, a condition where urine output is reduced but still above the threshold for anuria. Oliguria can be a sign of mild to moderate kidney impairment or temporary conditions such as shock or severe dehydration.

**Option B:** 3L is a normal or even increased urine output, which would not be classified as anuria. In fact, this volume of urine is typically produced by a healthy individual over a 24-hour period.

**Option D:** 1000ml is also within the normal range for urine output, particularly for a 24-hour period. This volume would not indicate anuria, but rather a normal or even increased urine production.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that anuria often requires prompt medical attention, as it can be a sign of severe kidney dysfunction or other underlying conditions. Early recognition and treatment of the underlying cause are essential to prevent further complications and potential kidney failure.

**✓ Correct Answer: C. 100ml**