## Core Concept
The shelf life of blood and its components is crucial for transfusion medicine. CPD stands for Citrate Phosphate Dextrose, which is an anticoagulant solution used for preserving whole blood and red blood cells. Understanding the shelf life helps in managing blood inventory and ensuring patient safety.

## Why the Correct Answer is Right
CPD (Citrate Phosphate Dextrose) is an anticoagulant solution that allows whole blood or red blood cells to be stored for a certain period. The shelf life of CPD blood is **35 days**. This duration is based on the ability of the CPD solution to maintain the viability and function of red blood cells during storage. The solution provides nutrients and helps in maintaining the pH, which is essential for preserving the cells.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not specify the correct duration.
- **Option B:** This option suggests a shorter shelf life, which is not accurate for CPD blood.
- **Option D:** This option suggests a longer shelf life, which exceeds the recommended storage duration for CPD blood.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the addition of adenine to CPD, forming CPDA-1, extends the shelf life to **42 days**. However, standard CPD without adenine has a shelf life of **35 days**. This distinction is crucial for managing blood supplies effectively.

## Correct Answer Line
**Correct Answer: C. 35 days**