## Core Concept
The patient's presentation of macroglossia (enlarged tongue) and atrophy of tongue papillae, along with a low hemoglobin (Hb) level of 11.5 and a high Mean Corpuscular Volume (MCV) of 100, suggests a diagnosis of megaloblastic anemia. This condition is characterized by the production of abnormally large red blood cells that are not able to function properly, and it often results from a deficiency in either vitamin B12 or folate.

## Why the Correct Answer is Right
The next step in investigating this patient would involve confirming the cause of the megaloblastic anemia. Given that both vitamin B12 and folate deficiency can cause megaloblastic anemia, it is crucial to determine which one is deficient. Vitamin B12 deficiency can lead to neurological complications if not treated promptly, making its diagnosis critical. Therefore, checking the levels of vitamin B12 and folate is essential.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but typically, incorrect options might include immediate treatment without diagnosis, or testing for conditions not directly related to megaloblastic anemia.
- **Option B:** Similarly, without specifics, if this option does not directly relate to assessing vitamin B12 and folate levels or other directly relevant diagnostic steps for megaloblastic anemia, it would be incorrect.
- **Option C:** If this option suggests a treatment approach without diagnostic confirmation (e.g., starting vitamin B12 or folate supplementation without measuring levels), it would be premature.
- **Option D:** If this option does not align with the diagnostic pathway for megaloblastic anemia (e.g., checking for vitamin B12 and folate levels), it would be incorrect.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that megaloblastic anemia can present with neurological symptoms if due to vitamin B12 deficiency, such as neuropathy or cognitive changes. Therefore, it's crucial to differentiate between vitamin B12 and folate deficiency. A useful mnemonic is "B12 - Brain, Blood, and Body" to remember its wide range of effects.

## Correct Answer: B.