**Core Concept:** Stored blood undergoes various changes in red blood cells (RBCs) due to the loss of oxygen, release of mediators, and exposure to various factors during storage.

**Why the Correct Answer is Right:** The correct answer, **D. Decrease in HbF and Increase in HbA2**, indicates that the stored blood undergoes changes in the type of hemoglobin present in RBCs. Hemoglobin (Hb) forms are categorized as fetal (HbF), adult (HbA), and abnormal (HbA2). During storage, there is a decrease in fetal hemoglobins (HbF) and an increase in adult hemoglobin (HbA2). This shift in hemoglobin forms is attributed to the oxygen-deprivation and exposure to various storage conditions.

**Why Each Wrong Option is Incorrect:**

A. **Increase in HbF**: This is incorrect because fetal hemoglobin is replaced by adult hemoglobin during storage, and the question states a decrease in fetal hemoglobin levels.

B. **Decrease in HbA2**: This is also incorrect, as the question indicates an increase in HbA2, which is a minor abnormality hemoglobin.

C. **Decrease in HbA**: This is incorrect because HbA represents the adult hemoglobin, and the question states an increase in HbA2, not HbA.

**Clinical Pearl:**

Understanding these changes in hemoglobin forms helps in differentiating between normal stored blood and blood with potential abnormalities. For instance, an increase in HbF might suggest a certain genetic disorder, while an increase in HbA2 could indicate a higher prevalence of alpha-thalassemia in the blood.

**Correct Answer:** **D. Decrease in HbF and Increase in HbA2**

**Explanation:** During the storage process, the oxygen deprivation and storage conditions lead to a decrease in fetal hemoglobin (HbF) and an increase in adult hemoglobin (HbA2). This shift in hemoglobin forms is a normal physiological response to store blood efficiently and ensure survival under oxygen-deprived conditions.