An organic compound 'A' is heated with fuming nitric acid in a Carius tube. The resulting acid is treated with magnesia mixture to yield a precipitate, which on ignition forms a stable pyrophosphate compound. The compound 'A' could be:

Step-by-step solution

Heating an organic compound with fuming nitric acid oxidizes the phosphorus present in it to phosphoric acid. On adding magnesia mixture, a precipitate of magnesium ammonium phosphate is formed, which on ignition yields magnesium pyrophosphate ( Mg ₂ P ₂ O ₇ ). This is the standard test for the estimation of phosphorus. Among the given options, a nucleotide contains a phosphate group and hence contains phosphorus. Cysteine contains sulphur, chloramphenicol contains chlorine, and thyroxine contains iodine. Answer: A