**Core Concept**
Hemoglobin is a crucial protein in red blood cells responsible for oxygen transport. Various forms of hemoglobin exist, each with distinct properties and functions. Understanding the types of hemoglobin is essential for diagnosing and managing various blood disorders.

**Why the Correct Answer is Right**
Hemoglobin A2 (HbA2) is a minor component of adult hemoglobin, making up approximately 2-3.5% of the total hemoglobin. However, it is not typically found in fetal hemoglobin, which is the main form of hemoglobin in the fetus. Fetal hemoglobin (HbF) is composed of two alpha-globin chains and two gamma-globin chains, whereas adult hemoglobin (HbA) consists of two alpha-globin chains and two beta-globin chains. HbA2 is a variant of adult hemoglobin with two alpha-globin chains and two delta-globin chains.

**Why Each Wrong Option is Incorrect**
* **Option B:** Fetal hemoglobin (HbF) is the main form of hemoglobin in the fetus, but it is not the correct answer because we are looking for a type of hemoglobin not normally found in adult erythrocytes.
* **Option C:** Hemoglobin A (HbA) is the main form of hemoglobin in adult erythrocytes, so it is not the correct answer.

**Clinical Pearl / High-Yield Fact**
Fetal hemoglobin (HbF) has a higher affinity for oxygen than adult hemoglobin, allowing it to pick up oxygen from the maternal bloodstream and transport it to the fetus.

**Correct Answer:** D. Fetal hemoglobin.