Review Question - QID 109723

A 32-year-old man presents to a mission hospital in Cambodia because he has had difficulty walking from his village to the market. He says that he has always been healthy other than occasional infections; however, over the last year he has been having numbness in his hands and feet. Furthermore, he has noticed weakness, muscle wasting, and pain in his lower extremities. The only change he can remember is that after having a poor harvest last year, he and his family have been subsisting on white rice. Physical exam reveals normal skin color and decreased deep tendon reflexes. The most likely cause of this patient's symptoms is associated with which of the following enzymatic reactions?