Okay, let's tackle this question about HbM and the position of the point mutation. First, I need to recall what HbM is. HbM is a variant of hemoglobin, right? So hemoglobinopathies are caused by mutations in the globin genes. HbM specifically is associated with abnormal hemoglobin that doesn't bind oxygen properly, leading to cyanosis even when oxygen saturation is normal.

The question is asking about the position of the point mutation. I remember that different hemoglobin variants have mutations in specific positions. For example, HbS (sickle cell) is a mutation in the beta-globin gene at position 6, changing glutamic acid to valine. But HbM is different. I think HbM mutations are in the alpha or beta chain, but where exactly?

Wait, there are several types of HbM. For instance, Hb M Boston is a beta-globin mutation at position 62, where histidine is replaced by tyrosine. Another type, Hb M Hyde Park, is a beta-globin mutation at position 63, valine to methionine. These mutations affect the heme iron's ability to bind oxygen, causing methemoglobinemia. So the position would be in the beta-globin gene at those specific sites.

The options here are missing, but the correct answer is likely to be a beta-globin chain with a specific position. If the options were, say, beta-62 or beta-63, then those would be correct. The other options might refer to alpha-globin positions or other positions in the beta chain that are not associated with HbM. For example, HbA is the normal adult hemoglobin with alpha and beta chains, and mutations elsewhere might cause other conditions like HbC or HbE, but not HbM.

I need to make sure that the explanation covers why HbM mutations are in the beta-globin gene at those positions, affecting the heme pocket. Also, distinguishing HbM from other types like HbS or HbC is important. The key point is that the mutation in HbM leads to methemoglobinemia, which is different from the structural changes in HbS causing sickling.

**Core Concept**
HbM (Hemoglobin M) variants result from **point mutations in the beta-globin gene (HBB)** at positions **62 or 63**, altering amino acids in the heme-binding pocket. These mutations stabilize the ferric (Fe³⁺) state of heme, leading to **methemoglobinemia** with cyanosis despite normal oxygen saturation.

**Why the Correct Answer is Right**
The correct answer identifies **beta-globin chain position 62 or 63** as the site of mutation. For example, HbM Boston involves **beta62His→Tyr**, while HbM Hyde Park is **beta63Val→Met**. These substitutions disrupt the heme pocket, preventing oxygen binding and causing persistent methemoglobin levels (>1.5 g/dL), which bind brownish methemoglobin in red cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Alpha-globin mutations (e.g., HbS at beta6) cause sick

✓ Correct Answer: B. a chain, 87th codon, Histidine - Tyrosine