Which of the following has highest affinity for Hb?

**Core Concept**
Carbon monoxide (CO) has a higher affinity for hemoglobin (Hb) than oxygen, leading to the formation of carboxyhemoglobin (COHb). This is due to the structural similarity between CO and oxygen, allowing CO to bind to the heme group with a much higher affinity.

**Why the Correct Answer is Right**
CO binds to the Fe2+ ion in the heme group of Hb, forming a stable complex with a higher affinity than oxygen. This results in the displacement of oxygen from Hb, leading to tissue hypoxia. The affinity of CO for Hb is approximately 210-240 times higher than that of oxygen. The high affinity of CO for Hb is due to its ability to form a strong covalent bond with the iron atom in the heme group.

**Why Each Wrong Option is Incorrect**
**Option A:** Oxygen has a lower affinity for Hb compared to CO.
**Option B:** Nitric oxide (NO) has a higher affinity for soluble guanylate cyclase than for Hb.
**Option C:** Carbon dioxide (CO2) does not bind directly to Hb.

**Clinical Pearl / High-Yield Fact**
CO poisoning is a significant clinical concern, particularly in enclosed spaces with poor ventilation. The high affinity of CO for Hb can lead to severe tissue hypoxia, even at low concentrations of CO.

**Correct Answer: C. Carbon dioxide does not bind directly to Hb.**