Most difficult criterion to establish Causal Association in aetiology of a disease is ?

Ans. is 'c' i.e., Specificity of association Specificity of association The concept of specificity implies a one to one relationship, i.e., disease under study is caused only by risk factor under study and risk factor under study causes only one disease which is under study. It is the weakest criteria of causal association because : ? i) Most diseases are due to multiple factors with no possibility of one to one relationship, e.g., lung cancer is associated with smoking, asbestos, nickle and air pollution. ii) A single cause can give rise to more than one disease, e.g., smoking can cause, Ca lung, Bronchitis, Bladder Ca, CHD, peptic ulcer. So, o Requirement of specificity is the most difficult criteria to establish. o But lack of specificity does not negate the causal relationship because : ? i) Though single cause can cause more than one disease, it may be associated in causation of disease under study, e.g., smoking is associated with so many diseases but it has been proved as the most impoant risk factor in causation of lung cancer. ii) Though most of the disease are due to multiple factor, disease under study may be associated with the risk factor under study, e.g., Lung cancer has multiple etiological risk factors, but smoking has been proved to be the most impoant risk factor. So, specificity suppo causal interpretation but lack of specificity does not negate it. Biological plausibility It implies biological credibility to the association, that is the association agrees with current understanding of the response of cells, tissues organs and system to stimuli. o For example, positive association of food intake with GI cancer is biological plausable (food enters it and causes cellular response), while positive association of food intake and skin cancer is not biological plausible. Option 'a & b' have already been explained.