Explanation
- A
Haemoglobin
Haemoglobin is the normal oxygen-carrying pigment of blood. Blue-baby syndrome is not caused by the mere presence of haemoglobin; it is caused when haemoglobin is converted into a form that cannot carry oxygen properly. This option is not the key.
- B
Methemoglobin
Methemoglobin is the official key. In blue-baby syndrome (methaemoglobinaemia) iron in haemoglobin is oxidised to the ferric state, producing methemoglobin, which cannot bind oxygen effectively. Infants then appear cyanosed. Excess nitrate in water is a common environmental trigger, but the pigment present in blood that defines the syndrome is methemoglobin. That is why (b) is correct.
- C
Lead
Lead in blood causes lead poisoning and anaemia, not the classic nitrate-linked blue-baby picture named in this item. This option is therefore wrong.
- D
Nitrate
Nitrate in drinking water is an important cause that leads to methemoglobin formation in infants, but the question asks what is present in the blood. The blood pigment is methemoglobin, not nitrate as such. This option is not the key.
Summary. Official key is (b) Methemoglobin. Blue-baby syndrome is methaemoglobinaemia: oxygen delivery fails because methemoglobin cannot carry oxygen. Nitrate contamination of water is a frequent cause, especially in bottle-fed infants, but the blood finding is methemoglobin. Normal haemoglobin and lead do not name that finding. Only (b) is correct.
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