**Core Concept**
Vitamin B12 plays a crucial role in the synthesis of myelin and the maintenance of the nervous system. A deficiency in Vitamin B12 can lead to a range of neurological and hematological manifestations.

**Why the Correct Answer is Right**
In the context of Vitamin B12 deficiency, the expected change is an increase in the levels of methylmalonic acid (MMA) and homocysteine. This occurs due to the impaired activity of methylmalonyl-CoA mutase and methionine synthase, which are dependent on Vitamin B12 as a cofactor. Elevated MMA and homocysteine levels are sensitive indicators of Vitamin B12 deficiency and can be used to diagnose this condition.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because, although anemia is a potential consequence of Vitamin B12 deficiency, it is not the most specific or sensitive indicator of this condition.
* **Option B:** Incorrect because, while elevated levels of homocysteine can be seen in Vitamin B12 deficiency, they are not exclusive to this condition and can also be elevated in other disorders, such as folate deficiency.
* **Option C:** Incorrect because, although Vitamin B12 deficiency can lead to neurological symptoms, such as numbness and weakness, these symptoms are not as specific or sensitive as the biochemical changes in MMA and homocysteine levels.

**Clinical Pearl / High-Yield Fact**
In patients with suspected Vitamin B12 deficiency, it is essential to measure both MMA and homocysteine levels, as these biomarkers can provide a more accurate diagnosis than isolated measurements of homocysteine.

**Correct Answer:** D. Elevated levels of methylmalonic acid (MMA) and homocysteine.