The amino acid that gives a red-blood colour on treating its sodium fusion extract with sodium nitroprusside is

Question

Accepted Answer

D. 1. Recall the purpose of sodium fusion extract.

Sodium fusion converts the nitrogen, sulphur and halogens present in an organic compound into simple, detectable ionic forms such as $NaCN$, $Na_2S$ and sodium halides.

2. Recall what sodium nitroprusside detects.

Sodium nitroprusside test specifically detects the presence of **sulphur** in the compound, normally giving a violet colour when sulphide ($S^{2-}$) alone is present.

3. Recall the special case when both nitrogen and sulphur are present together.

Given that the molecule contains both N and S, they can combine during fusion, therefore the test instead gives a characteristic **blood-red colour** rather than the usual violet.

4. Match this behaviour to the given amino acids.

Among serine, leucine, threonine and methionine, only **methionine** contains a sulphur atom (in its side chain) along with the nitrogen already present in every amino acid.

Hence, the answer is **D. methionine**.