Lactic acidosis  in  thiamine  deficiency  is  due  to which enzyme dysfunction?
In Thiamine deficiency, PDH reaction is defective. So, pyruvate is converted to lactic acid.
Causes of inhibition of PDH leading to lactic acidosis:

Inherited PDH deficiency
Thiamine deficiency
Alcoholics due to thiamine deficiency
Arsenite and Mercury poisoning

Harper 30th edition
Phosphoenol Pyruvate Carboxykinase
Pyruvate Dehydrogenase
Pyruvate Carboxylase
Aldolase