Heavy ETOH Plus a Metallic Taste: Clues to What Deficiency?

This patient's presentation also included a pruritic rash around the nasolabial region and inguinal folds. What type of deficiency would you test for?

A 60-year-old man presents complaining of a metallic taste in his mouth of several weeks' duration. He has a long-standing history of heavy alcohol consumption. On examination, he has a pruritic scaly rash around his nasolabial region and inguinal folds.

Which of the following deficiencies is the most likely cause of this patient’s presentation?