Diagnosis of carcinoid tumors is done by:

Answer is A (5HIAA): The diagnosis of typical carcinoid tumors is done by measurement of 5HIAA levels. Diagnosis of Carcinoid Tumors (Harrison) The diagnosis of carcinoid syndrome relies on measurement of urinary or plasma serotonin or its metabolites in the urine. Typical carcinoid syndrome (high level of serotonin) is characterized by high levels of 5HIAA. The measurement of 5HIAA is most frequently used for typical carcinoid syndrome. Most physicians' only use urinary 5HIAA levels / excretion rates however plasma and platelets serotonin levels if available provide additional information. Platelet serotonin levels (5-HT) are more sensitive than urinary 5HIAA levels but are not generally available. Atypical carcinoid syndromes (low levels of serotonin) may have normal or minimally elevated 5HIAA levels. In such patient's urinary levels of other tryptophan metabolites such as 5HTP should be measured. Elevated levels of urinary 5 HTP is suggestive of an atypical carcinoid syndrome.