## **Core Concept**
CPDA (Citrate Phosphate Dextrose Adenine) buffer is a solution used in blood banks for preserving blood. It helps maintain the viability of red blood cells during storage by providing nutrients and maintaining the pH.

## **Why the Correct Answer is Right**
The CPDA buffer is specifically designed to extend the shelf life of whole blood or red blood cells. The addition of adenine in the CPDA solution allows for the synthesis of ATP through the purine salvage pathway, which is crucial for maintaining red blood cell viability. With CPDA, the shelf life of blood can be extended up to 35 days.

## **Why Each Wrong Option is Incorrect**
* **Option A:** 28 days - This is incorrect because while some blood preservation solutions have a shelf life of 28 days, CPDA allows for a longer shelf life.
* **Option B:** 21 days - This is incorrect as it underestimates the shelf life provided by CPDA.
* **Option D:** 42 days - This is incorrect because, although some newer solutions like CPDA-1 or AS-1 (Adsol) allow for storage up to 42 days, standard CPDA typically allows for storage up to 35 days.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the shelf life of blood can vary depending on the preservative solution used. CPDA-1, for instance, allows for storage of blood for up to 35 days, making it essential to label and track blood products accurately to ensure they are used within their safe storage period.

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