The most appropriate diagnostic test is to measure the methylmalonic acid level. This patient likely has cobalamin (vitamin B₁₂) deficiency, which she is at risk for based on following a vegan diet for several years. Compared with folate deficiency, which develops within weeks, cobalamin deficiency develops over months to years. She also has neurologic findings typical of and concerning for cobalamin deficiency. If left untreated, neurologic function could continue to decrease. A macrocytic anemia is characteristic of cobalamin deficiency, and the peripheral blood smear shows a typical hypersegmented neutrophil, a result of ineffective myelopoiesis. In many patients with suspected cobalamin deficiency, vitamin B₁₂ levels will be in the normal range, and further testing is indicated to obtain better sensitivity. Methylmalonic acid is elevated in 98% of people with cobalamin deficiency; therefore, this would be a sensitive and specific test to determine deficiency.

Cobalamin deficiency can occasionally present with pancytopenia, which could prompt a bone marrow examination; however, a bone marrow biopsy is not needed to diagnose suspected cobalamin deficiency. The patient's neutropenia will likely resolve as the cobalamin deficiency is corrected.

This patient is unlikely to have isolated folate deficiency considering her neurologic symptoms, which are commonly seen with cobalamin, but not folate, deficiency. Testing the methylmalonic acid level concomitantly rules out folate deficiency and diagnoses cobalamin deficiency, because methylmalonic acid levels will be normal in cases of folate deficiency.

Homocysteine levels are elevated in cobalamin deficiency, but they can also be elevated in folate deficiency, so checking the homocysteine level is not the best test to perform. Evaluating the methylmalonic acid level will more accurately provide a diagnosis.