hi this is dr.subhadeep.thanks for the opportunity to help you. presence of one painless lymph node in the posterior triangle of neck doesn t have to be cancer always and that to as per your description it has only lasted for one week. some additional information will be beneficial like your age, sex, smoker or alcoholic and little bit of elaboration of your symptoms like history of sore throat,fever, weight loss,evening rise of temperature.weight loss (T.B, Malignancy)should be significant more than 10 kg in 6 months,any scalp infection like seberrhoeic dermatitis and lastly malignancy:- Hodgkin s lymphoma,NHL,secondaries due to known,unknown or occult primary including visceral malignancies. however chances of malignancy should be considered only at the last. most common possible diagnosis are:- sore throat,T.B lymphadenitis,kikuchi fugimoto s disease(rare),seberrhoeic dermatitis,Malignancy. an FNAC should be tried first and if it confirms T.B then start ATT for 6 months.if FNAC is inconclusive do an excision biopsy.if rarest of rarest circumstances it turns out to be malignant then Triple or Panendoscopy to find the primary followed by definitive treatment of primary with MRND but in your case it will be rarest possibility. kindly get Total count,E.S.R,Mantoux test to rule out T.B and chest x ray p/a view.this will be very helpful to come to diagnosis. kindly let me know if the consultation was beneficial. regards