## **Core Concept**
The shelf life of blood in a blood bank is determined by the type of anticoagulant-preservative solution used. CPDA (Citrate-Phosphate-Dextrose-Adenine) buffer is one such solution that helps in maintaining the viability of red blood cells during storage.

## **Why the Correct Answer is Right**
CPDA buffer is specifically designed to provide nutrients and maintain the pH of stored blood, thereby extending the shelf life of red blood cells. The CPDA-1 solution, for example, allows for the storage of whole blood or red blood cells for up to 35 days. This is because it contains citrate as an anticoagulant, phosphate as a buffer, dextrose as an energy source, and adenine, which is a substrate for ATP synthesis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** 28 days. This is incorrect because while some anticoagulant solutions may have a shelf life of 28 days, CPDA buffer solutions are known for extending the storage life beyond this.
- **Option B:** 21 days. This is incorrect as it underestimates the shelf life provided by CPDA buffer solutions.
- **Option D:** 42 days. This is incorrect because, although there are solutions like CPDA-2 with longer storage capabilities, CPDA-1, a commonly referenced solution, typically allows storage for 35 days.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the shelf life of blood can vary depending on the anticoagulant-preservative solution used. CPDA-1 allows for storage of blood for 35 days, making it a valuable solution for blood banks.

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