## Core Concept
Hemoglobin is a protein in red blood cells that carries oxygen to different parts of the body. It contains four polypeptide chains (globins) and four heme groups, each of which contains one iron atom. The iron in hemoglobin is crucial for its oxygen-carrying capacity.

## Why the Correct Answer is Right
One gram of hemoglobin contains approximately 3.4 milligrams of iron. This is because each molecule of hemoglobin contains four iron atoms, and the molecular weight of hemoglobin is approximately 64,000 g/mol. Given that the atomic weight of iron is about 56 g/mol, the calculation yields that 1 gram of hemoglobin contains about 3.4 milligrams of iron.

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests a much lower amount of iron in 1 gram of hemoglobin, which does not align with the known composition of hemoglobin.
- **Option B:** Similarly, this option suggests an incorrect amount of iron, not matching the established fact that 1 gram of hemoglobin contains about 3.4 milligrams of iron.
- **Option D:** This option suggests a higher amount of iron than what is actually present in 1 gram of hemoglobin.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the amount of iron in hemoglobin can be used to calculate the total iron content in the body based on the total amount of hemoglobin. This is particularly relevant in conditions like anemia, where iron deficiency is a common cause.

## Correct Answer: .