**Core Concept**
Hemoglobin is a protein in red blood cells responsible for binding and transporting oxygen from the lungs to the body's tissues. It also plays a role in buffering carbon dioxide, a byproduct of cellular metabolism, through the Bohr effect.

**Why the Correct Answer is Right**
Hemoglobin binds to oxygen through a process called oxygenation, which involves the binding of oxygen to the heme group in hemoglobin. This is facilitated by the high affinity of the iron atom in the heme group for oxygen. Hemoglobin also binds to carbon dioxide (CO2) through a reaction with carbonic anhydrase, which converts CO2 to bicarbonate (HCO3-). Additionally, hemoglobin binds to carbon monoxide (CO) in a similar manner to oxygen, although with much higher affinity. However, hemoglobin does not bind to hydrogen cyanide (HCN), which is a potent inhibitor of cellular respiration. HCN binds to cytochrome c oxidase in mitochondria, preventing the electron transport chain from functioning, and ultimately leading to cellular asphyxiation.

**Why Each Wrong Option is Incorrect**
**Option A:** Oxygen is a normal substrate for hemoglobin binding, which is essential for oxygen transport in the body.
**Option B:** Carbon dioxide is another substrate for hemoglobin binding, which is important for maintaining acid-base balance.
**Option C:** Carbon monoxide has a high affinity for hemoglobin, similar to oxygen, and can displace oxygen from hemoglobin, leading to tissue hypoxia.

**Clinical Pearl / High-Yield Fact**
It's essential to note that carbon monoxide poisoning can occur through inhalation of CO, which binds to hemoglobin and displaces oxygen, leading to tissue hypoxia and potential organ damage.

**✓ Correct Answer: D. HCN. Hydrogen cyanide (HCN) does not bind with hemoglobin.**