## **Core Concept**
Fetal red blood cells (RBCs) have distinct characteristics that differentiate them from adult RBCs, particularly in terms of their hemoglobin, cell size, and lifespan. These differences are crucial for adapting to the fetal environment.

## **Why the Correct Answer is Right**
Fetal RBCs contain **fetal hemoglobin (HbF)**, which has a higher affinity for oxygen compared to adult hemoglobin (HbA). This allows for the efficient uptake of oxygen from the mother's bloodstream. Fetal RBCs are also **larger** and have a **longer lifespan** compared to adult RBCs. The presence of **nucleus** in fetal RBCs at certain stages is another distinctive feature.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Typically, statements about fetal RBCs being larger and having a longer lifespan are true, which might relate to options not directly provided but commonly known facts.
- **Option B:** If an option suggests that fetal RBCs have a similar lifespan to adult RBCs, this would be incorrect because fetal RBCs generally have a longer lifespan.
- **Option C:** If an option mentions that fetal RBCs contain HbF, this would be correct and not the answer we're looking for.
- **Option D:** Without specific details on each option, a common incorrect statement could be that fetal RBCs have a lower affinity for oxygen compared to adult RBCs, which contradicts the known higher affinity of HbF for oxygen.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **Kleihauer-Betke test** can be used to detect fetal-maternal hemorrhage by identifying fetal RBCs in the maternal circulation based on their resistance to acid elution of adult hemoglobin.

## **Correct Answer:** D. Adult hemoglobin.