**Question:** Feature of fetal RBC in adult -

A. Increased nucleus size
B. Absence of hemoglobin A
C. Lower MCV (mean corpuscular volume)
D. Presence of fetal hemoglobin (HbF)

**Core Concept:**
The key concept being tested here is the differences in red blood cell (RBC) characteristics between fetal and adult RBCs. In the fetal stage, RBCs exhibit some unique features which are distinct from the adult RBCs.

**Why the Correct Answer is Right:**
In adults, the predominant hemoglobin is hemoglobin A (HbA), which is a mature form of hemoglobin. However, in fetuses, the predominant hemoglobin is fetal hemoglobin (HbF), a less efficient oxygen-carrying form. The presence of HbF in adult RBCs is a key feature of fetal RBCs, which is why option D is correct.

**Why Each Wrong Option is Incorrect:**
A. Increased nucleus size is a characteristic of fetal RBCs, but it is not unique to fetal RBCs and can also be seen in some adult RBCs with certain conditions (e.g., polycythemia).
B. Absence of hemoglobin A is incorrect, as adults do have HbA. However, it is not a feature specific to fetal RBCs.
C. Lower mean corpuscular volume (MCV) is also not a unique feature of fetal RBCs, as it can be seen in various pathological conditions like iron deficiency anemia or macrocytic anemia.

**Clinical Pearl:**
The presence of fetal hemoglobin (HbF) in adult RBCs is a significant finding for doctors, as it can be indicative of certain diseases, such as sickle cell anemia or thalassemia, where there is a significant increase in fetal hemoglobin production. This can assist in making a correct diagnosis and guiding treatment strategies.

**Correct Answer:** D. Presence of fetal hemoglobin (HbF) in adult RBCs.