关键词: 维生素B12缺乏, 晕厥, 全血细胞减少, 脊髓亚急性联合变性

Abstract: Vitamin B12 deficiency is a nutritional deficiency in the world whose incidence is far higher than estimated，especially in the elderly.However，due to insufficient awareness of and attention to it，frequently the patient does not obtain a definite diagnosis until he develops multisystem lesions.In order to improve clinicians' awareness of illnesses associated with vitamin B12 deficiency，we reported a case of multisystem lesions associated with vitamin B12 deficiency and reviewed the relative studies，then discussed metabolic pathways of folic acid and vitamin B12，and illnesses caused by the deficiency of folic acid and vitamin B12.Moreover，from the patient-centered systemic perspective，clinical problem-oriented perspective and evidence-based clinical decision-making perspective，we made the final diagnosis after carefully excluding other conditions，which well interprets the essential ideas in general practice as well as the monistic principle，providing guidance for clinicians.

Key words: Vitamin B12 deficiency, Syncope, Pancytopenia, Subacute combined degeneration