A

Riboflavin is the FAD co-factor for erythrocyte glutathione reductase; low enzyme activity (or a high activation coefficient) is a recognized biomarker of riboflavin deficiency, which also presents with angular stomatitis and magenta/fissured tongue. Peripheral neuropathy can accompany prolonged deficiency. [Prentice & Bates, 1981, PMID 7470438; Parkington et al., 2023, PMID 37988268]