**Question:** Hb A2 concentration in thalassemia trait is –

**Core Concept:**
Hemoglobin (Hb) is a protein found in red blood cells (RBCs) that carries oxygen from the lungs to the body's tissues and transports carbon dioxide back to the lungs. Hemoglobin is composed of four chains: two alpha (HbA2) and two beta (HbA). HbA2 is the second most abundant type of hemoglobin in normal individuals, with a concentration of about 3-5%. Thalassemia is a group of inherited blood disorders characterized by reduced production of HbA2 and other hemoglobin types due to mutations in the globin genes.

**Why the Correct Answer is Right:**
In thalassemia trait, there is a reduction in the production of HbA2 due to a mutation in the alpha-globin gene. This leads to a decrease in the concentration of HbA2 in the RBCs. The correct answer is D (3-5%), as it represents the normal range for HbA2 concentration in healthy individuals.

**Why Each Wrong Option is Incorrect:**
A. This option (8%) is too high, reflecting an increase in HbA2 concentration which is not present in thalassemia trait.
B. This option (0%) is too low, as thalassemia trait results in a reduction in HbA2 concentration, not absence.
C. This option (6%) is slightly lower than the correct answer but still within the normal range, making it incorrect for thalassemia trait.

**Clinical Pearl:**
Understanding the HbA2 concentration in thalassemia trait is crucial for differentiating it from other disorders with altered hemoglobin levels. A higher HbA2 concentration would be indicative of another condition, such as beta-thalassemia major or alpha-thalassemia. Regular monitoring of HbA2 levels helps in managing and following the progression of the disease in thalassemia patients.