**Question:** Each milliliter of red cells contains ....... mg of elemental iron:

A. 2.5
B. 3.6
C. 6.5
D. 7.2

**Correct Answer:**

**Core Concept:** Elemental iron is the basic form of iron present in the body, which is essential for the synthesis of hemoglobin and myoglobin. Hemoglobin is a protein in red blood cells (RBCs) that carries oxygen from the lungs to the tissues and returns carbon dioxide.

**Why the Correct Answer is Right:**

In RBCs, most of the iron is present in the form of heme, which is a part of the protein hemoglobin. Hemoglobin has four iron atoms (in the form of heme groups) per molecule, contributing to a total of approximately 4 mg of iron per 1 million RBCs. Since the normal hemoglobin concentration in human blood is around 15 grams per deciliter (g/dL), and the adult human has about 50-60 grams of hemoglobin, the total amount of iron in the body is around 3-4 grams.

**Why Each Wrong Option is Incorrect:**

A. 2.5 mg: This is too low, as 4 mg of iron per 1 million RBCs is closer to the correct answer.
B. 3.6 mg: This is also too low, as the actual amount is around 4 mg.
C. 6.5 mg: This is too high, as the actual amount is around 4 mg.
D. 7.2 mg: This is too high, as the actual amount is around 4 mg.

**Clinical Pearl:**

Iron is an essential element for human beings as it plays a vital role in oxygen transport and storage, DNA synthesis, and cellular respiration. A deficiency of iron can lead to anemia, which can be caused by various factors such as poor dietary intake, heavy menstruation, or gastrointestinal bleeding.

**Why Each Wrong Option is Incorrect:**

A. 2.5 mg: This is too low, as the actual amount is around 4 mg.
B. 3.6 mg: This is too low, as the actual amount is around 4 mg.
C. 6.5 mg: This is too high, as the actual amount is around 4 mg.
D. 7.2 mg: This is too high, as the actual amount is around 4 mg.