**Core Concept**
Fetal red blood cells (RBCs) have distinct adaptations to meet the unique needs of the fetus. These adaptations enable the fetus to thrive in the low-oxygen environment of the womb. Key differences include altered hemoglobin structure, decreased 2,3-bisphosphoglycerate (2,3 DPG) levels, and changes in RBC lifespan and volume.

**Why the Correct Answer is Right**
2,3 DPG is an important regulator of oxygen delivery to tissues. In adult RBCs, 2,3 DPG binds to hemoglobin, reducing its oxygen affinity. This allows for more efficient oxygen release to tissues. In contrast, fetal RBCs have decreased 2,3 DPG levels, which results in a higher oxygen affinity of hemoglobin. This adaptation ensures that the fetus receives a constant supply of oxygen from the mother's bloodstream.

**Why Each Wrong Option is Incorrect**
**Option B:** Carbonic anhydrase is an enzyme involved in the regulation of pH in the blood. While it plays a crucial role in adult RBCs, there is no significant difference in carbonic anhydrase activity between fetal and adult RBCs.

**Option C:** Fetal RBCs actually have a longer lifespan than adult RBCs, not a decreased lifespan. This increased lifespan helps to meet the high metabolic demands of the growing fetus.

**Option D:** Fetal RBCs have a decreased RBC volume, not a high RBC volume. This decrease is due to the unique adaptations of fetal RBCs to the low-oxygen environment.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation is that newborns have a higher 2,3 DPG level than adults, which helps to regulate oxygen delivery to tissues after birth. This is an important consideration in neonatal resuscitation and the management of newborns with respiratory distress.

**✓ Correct Answer: A. Elevated 2,3 DPG**