Final answer:
A specific symptom associated with very low B12 is megaloblastic anemia, a disorder characterized by enlarged and immature red blood cells.
Step-by-step explanation:
A specific symptom associated with very low B12 is megaloblastic anemia. Megaloblastic anemia involves a deficiency of vitamin B12 and/or folate, which can result from diets deficient in these essential nutrients or a lack of meat or vegetable sources. Anemia is a disorder in which the blood has an inadequate volume of red blood cells, leading to symptoms such as weakness, fatigue, and in the case of megaloblastic anemia, enlarged and immature red blood cells.