**Core Concept**
Anemia is characterized by a reduction in red blood cell mass or hemoglobin concentration, leading to decreased oxygen delivery to tissues. Circulatory biomechanics plays a crucial role in understanding how the body compensates for this reduction. Cardiac output, the volume of blood pumped by the heart per minute, is a critical parameter that adjusts to meet the body's oxygen demands.

**Why the Correct Answer is Right**
In anemia, the body attempts to compensate for decreased oxygen delivery by increasing cardiac output. This is achieved through various mechanisms, including increased heart rate and stroke volume. The heart pumps more blood per minute to ensure that tissues receive adequate oxygenation. This compensatory mechanism is essential for maintaining tissue oxygenation and preventing tissue hypoxia. The increased cardiac output is mediated by the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system, which work together to increase blood pressure and cardiac output.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood viscosity is increased in anemia due to the relative increase in red cell mass, not decreased. In anemia, the body produces more erythropoietin, stimulating red blood cell production, which increases blood viscosity.
**Option B:** Blood viscosity is increased in polycythemia, not decreased. Polycythemia is characterized by an excessive increase in red blood cell mass, leading to increased blood viscosity.
**Option D:** Cardiac output is decreased in Beri-Beri, not increased. Beri-Beri is a disease caused by thiamine deficiency, leading to cardiac failure and decreased cardiac output.

**Clinical Pearl / High-Yield Fact**
In patients with anemia, the body's compensatory mechanisms can lead to increased cardiac workload and potential cardiac complications. It is essential to monitor cardiac function and adjust treatment accordingly to prevent cardiac failure.

**✓ Correct Answer: C. Cardiac output is increased in anemia**