Montelukast Sodium impurity: 1-?[[[(1R)?-?1-?[3-?[(1E)?-?2-?(7-?chloro-?2-?quinolinyl)?ethenyl]?phenyl]?-?3-?[2-?(1-?methoxy-?1-?methylethyl)?phenyl]?propyl]?thio]?methyl]?-Cyclopropaneacetic acid.