This is one of the easiest questions of CAT 18VA. The clue to the sequence lies in the pronoun "it', and in the phrase 'had come back', which suggests that it must have gone first, and then it must have come back. The pronoun 'it' refers to a disease, and is most likely to refer to the noun 'skin cancer'. Also, we must look for that sentence in which 'the skin cancer' must have gone back. Sentence 1 has the noun 'skin cancer' and says that the treatment had gone well. This connects sentence 1 with 3.13 is a pair. Statement 4 is likely to start the paragraph because it opens the idea by suggesting that something had started. So the idea goes like this: it started with a lump and no one knew what it was. Thus, 4 and 2 form a pair. After this must have come the diagnosis. Thus 4213 is the right sequence.