<prompt> Red blood cell transketolase level is a marker for deficiency of:Vitamin B1Vitamin B2Vitamin B6Vitamin B12  <response>: Vitamin deficiencies and enzyme markersMeasurement of RBC transketolase activity is a reliable disgnostic test to assess thiamine vitamin B1 deficiency.Assay of the enzyme RBC glutathione peroxidise will be useful in assessing riboflavin vitamin B2 deficiency.Xanthurenic acid, produced in high quantities is excreted in urine, which serves as a reliable index paicularly after tryptophan load test for vitamin B6 deficiency.The excretion of methylmalonic acid elevated in urine and estimation of serum B12 level are used to assess B12 deficiency. 