**Question:** One molecule of Hb can bind to a maximum number of

A. 2
B. 3
C. 4
D. 6

**Correct Answer:** D. 6

**Core Concept:** Hematology and Hemoglobin

Hemoglobin (Hb) is a protein found in red blood cells (RBCs) that carries oxygen from the lungs to the body's tissues and returns carbon dioxide back to the lungs for exhalation. Hemoglobin is made up of four polypeptide chains, two alpha (α) and two beta (β), which form a tetramer structure. The functional unit of hemoglobin is a dimer (two identical subunits), and each dimer can bind to one oxygen molecule. Since there are two dimers in a tetramer, each tetramer can bind to a maximum of two oxygen molecules.

**Why the Correct Answer is Right:**

Hemoglobin is a tetramer composed of two alpha and two beta chains. Each dimer (two identical subunits) within the tetramer has an oxygen-binding site, and the binding sites are functional units. Therefore, when considering the maximum number of oxygen molecules that one molecule of Hb can bind to, we should consider the number of functional units (dimers) present within the tetramer.

**Why Each Wrong Option is Incorrect:**

A. 2: This is incorrect because it only accounts for one oxygen-binding site (dimer) per tetramer.
B. 3: Similar to option A, this option only considers one binding site per tetramer.
C. 4: This is incorrect because it does not account for the fact that each dimer has one oxygen-binding site.
D. 6: This option is correct as it recognizes that a tetramer contains two oxygen-binding sites (dimers), allowing it to bind a maximum of six oxygen molecules.

**Clinical Pearl:**

Understanding the structure and function of hemoglobin is crucial for medical students to comprehend the oxygen-carrying capacity of red blood cells and the role of hemoglobin in oxygen transport. This knowledge is essential for interpreting blood test results, diagnosing hemoglobinopathies, and understanding the pathophysiology of anemia.