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

摘要： 维生素B12（VitB12）缺乏是一种全球性的营养素缺乏病，其发病率远比估计的高，尤其是老年人，但人们对其认识与重视程度不足，患者常出现多系统病变时才被诊断。本文报道1例VitB12缺乏引起多系统病变的患者，并通过文献复习，对叶酸、VitB12相关代谢通路与叶酸、VitB12缺乏相关疾病进行探讨，旨在提高临床医生对VitB12缺乏相关疾病的认识。同时本文在疾病的诊断过程中，坚持以患者为中心的系统思维，以问题为导向的临床诊断思维，以证据为基础的临床决策思维，鉴别诊断贯穿始终，对全科思维与疾病的一元论做了很好的诠释，相信对医务人员有较大的借鉴意义。

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