No, not D12 (the Detroit rap group - former member: Eminem), we're talking about B12 today. B12 deficiency is kind of neat...there are interesting causes and consequences to this condition. We will focus on causes here.
1. Decreased Intake: B12 is found primarily in animal products like meat, eggs, and milk. Strict vegans may be at risk.
2. Stomach-Related Conditions: Remember that B12 must bind to Intrinsic Factor (released from gastric parietal cells) to ultimately be absorbed in the terminal ilium. In Pernicious Anemia, autoantibodies attack intrinsic factor - this condition can be diagnosed with the Schillings test or by detecting autoantibodies. Autoimmune Chronic Atrophic Gastritis from autoanitbodies directed against parietal cells will also cause B12 deficiency. Also consider chronic gastritis, gastrectomy or gastric bypass as potential causes.
3. Intestinal Issues: Diphyllobothrium (Fish Tapeworm) is a massive worm...like-the-up-to-10-metres-long kind of massive, that can cause B12 malabsorption. Also consider diseases that will impair B12 absorption in the terminal ilium. Crohn's Disease is common here as is Lymphoma, and depending on the patients geographical past, think about Tuberculosis iliitis. Chronic pancreatitis is another common etiology.
4. Medication: Prolonged usage of proton pump inhibitors may cause B12 deficiency from impared gastric acid secretion, and metformin has been implicated in B12 deficiency from a calcium-mediated process in the ilium.
5. Rare inherited diseases: For example Imerslund-Grasbeck's syndrome, where there is impaired uptake of the B12-Intrinsic Factor complex in the terminal ilium.
Here is a neat case and a good approach to B12 deficiency.
Here is a terrific review on Pernicious Anemia from NEJM.
Here is the Rational Clinical Exam paper for splenomegaly from JAMA.
(Diphyllobothrium...aka Fish Tapeworm aka Gross)