A 68-year-old man comes to the clinic because he has tingling in his hands and forearms. His hands feel clumsy and he has begun to drop things, such as his keys and coffee cup. He is walking unsteadily. Over the past 4 months, he has noticed intermittent lower abdominal pain, decrease in appetite, fatigue, and shortness of breath when climbing stairs. History includes hypertension. He takes lisinopril as well as acetaminophen as needed. He does not smoke or drink.
On examination, weight is 68 kg (150 lb), a decrease from his usual weight of 74.8 kg (165 lb). He is pale, with icteric conjunctiva and skin. He has decreased sensation to light touch in his hands and feet and decreased vibratory sensation in a stocking pattern in his legs. Romberg test is positive (he sways and has near falls); the rest of the neurologic and physical examinations are normal.
WBC 7,000/μL with normal distribution
Hemoglobin 6.3 g/dL
RBC distribution width 18%
Mean corpuscular volume 124 fL
Mean corpuscular hemoglobin 42 pg/RBC
Mean corpuscular hemoglobin concentration 34 g/dL
Total bilirubin 3.2 mg/dL
Direct bilirubin 0.7 mg/dL
Lactate dehydrogenase 1,200 U/L
The remaining liver function tests are normal. Electrolytes, glucose, BUN, creatinine, creatine kinase, amylase, magnesium, calcium, and phosphorus are normal.
In order to establish the diagnosis, which of the following should be done next?
(A) CT of the abdomen
(B) MRI of the spine
(C) Bone marrow biopsy and aspirate
(D) Measurement of thyroid function
(E) Measurement of methylmalonic acid and homocysteine levels