Which vitamin deficiency is measured by a vitamin B12 test?

A vitamin B12 test is specifically designed to assess the levels of vitamin B12 in the blood, which directly measures whether there is a deficiency of this particular vitamin. Vitamin B12 is crucial for various bodily functions, including red blood cell production, nerve function, and DNA synthesis. A deficiency in vitamin B12 can lead to anemia and neurological issues, highlighting the importance of monitoring its levels in patients presenting symptoms that may be related to this deficiency.

The other options are unrelated to the vitamin B12 test. Vitamin A, C, and D deficiencies are evaluated using different specific tests that measure their respective levels in the body, not through the B12 assessment. Hence, the focus of the vitamin B12 test is exclusively on identifying a deficiency of vitamin B12 itself.