**Question:** Fetal Hb is different from HBA in that it is:

**Core Concept:** Hemoglobin (Hb) is a protein found in red blood cells that transports oxygen from the lungs to the body's tissues. During fetal life, two types of Hb are produced: fetal Hb (HbF) and adult Hb (HbA).

**Why the Correct Answer is Right:** Fetal HbF is composed of two alpha-globin chains (alpha2) and two gamma-globin chains (gamma4), while adult HbA is composed of two alpha-globin chains (alpha2) and two beta-globin chains (beta2). The difference lies in the type of gamma-globin chains present in fetal Hb.

**Why Each Wrong Option is Incorrect:**

A. **False**: Fetal Hb is different from HBA, not HbF.
B. **False**: HbA is composed of two beta-globin chains (beta2), not three.
C. **True**: Fetal Hb is different from HbA, but the explanation above explains why fetal Hb is different from HBA.
D. **False**: The difference lies in the type of gamma-globin chains, not the total number of globin chains.

**Clinical Pearl:** A decrease in fetal Hb levels after birth is a key feature in diagnosing certain genetic disorders, such as beta-thalassemia, which is caused by mutations in the beta-globin genes. Understanding the differences between fetal and adult hemoglobins is essential for accurate diagnosis and management of these conditions.

**Correct Answer:** C. Fetal Hb is different from HbA in terms of the type of gamma-globin chains present, which are gamma4 in fetal Hb and alpha2beta2 in adult HbA.