**Core Concept**
The H5N1 virus, also known as avian influenza, is a subtype of the influenza A virus. The global pandemic potential of this virus is influenced by its ability to infect humans and spread efficiently between individuals. The main factor determining the pandemic potential of a virus is its ability to undergo antigenic shift or drift, leading to significant changes in its surface proteins.

**Why the Correct Answer is Right**
The correct answer is that H5N1 does not have a major receptor binding site change, which is necessary for efficient human-to-human transmission. The H5N1 virus binds to the sialic acid receptors on the surface of human cells through its hemagglutinin protein. However, the receptor binding site of H5N1 is different from that of human-adapted influenza viruses, which are better suited to infect humans. This difference limits the ability of H5N1 to efficiently spread between humans and is a major reason why it has not caused a global pandemic.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while the H5N1 virus has a high mortality rate, it is not the primary reason why it has not become a global pandemic.
**Option B:** This option is incorrect because the H5N1 virus has been shown to be transmitted between humans in limited outbreaks, but this is not the primary reason why it has not caused a global pandemic.
**Option C:** This option is incorrect because the H5N1 virus has not undergone significant antigenic shift or drift, which would be necessary for it to become a global pandemic.

**Clinical Pearl / High-Yield Fact**
A key factor in determining the pandemic potential of a virus is its ability to undergo antigenic shift or drift, leading to significant changes in its surface proteins. This process can occur through reassortment or mutation, resulting in new strains of the virus that can more efficiently infect humans.

**Correct Answer:** D.