**Question:** One unit of fresh blood increases the Hb% concentration by -

**Core Concept:** Haemodilution is the temporary decrease in plasma proteins and electrolytes due to the infusion of fresh blood, which contains higher plasma proteins and electrolytes than the recipient's blood. Haemodilution leads to dilution of Hb% concentration, as Hb is mostly present in plasma.

**Why the Correct Answer is Right:** One unit of fresh blood, which contains higher plasma proteins and electrolytes than the recipient's blood, is infused. This leads to dilution of the recipient's plasma, including Hb. As a result, the Hb% concentration increases.

**Why Each Wrong Option is Incorrect:**

A. Haemoconcentration: This refers to the increase in plasma proteins and electrolytes due to fluid shifts from the interstitial space to the circulation, not dilution. It is the opposite of haemodilution and does not explain the increase in Hb% concentration after receiving fresh blood.

B. Haemoconcentration: Similar to Option A, this term describes an increase in plasma proteins and electrolytes due to fluid shifts, not dilution. It does not describe the observed increase in Hb% concentration after receiving fresh blood.

C. Haemoconcentration: This term is incorrect because it refers to an increase in plasma proteins and electrolytes due to fluid shifts, not dilution. It is the opposite of haemodilution and does not explain the increase in Hb% concentration after receiving fresh blood.

D. Haemodilution: This term describes the dilution of plasma proteins and electrolytes, including Hb, as a result of infusing fresh blood. This option correctly explains the increase in Hb% concentration after receiving fresh blood.

**Clinical Pearl:** The concept of haemodilution is crucial in understanding the effects of blood transfusions and is essential in clinical practice, especially for proper patient monitoring and management.

**Correct Answer:** D. Haemodilution: This term describes the dilution of plasma proteins and electrolytes, including Hb, as a result of infusing fresh blood. This option correctly explains the increase in Hb% concentration after receiving fresh blood.