A 71-year-old man is brought to the primary care clinic by his son, who reports that his f… — USMLE Step 3

A $71$ -year-old man is brought to the primary care clinic by his son, who reports that his father has become increasingly forgetful and fatigued over the past $6$ months. The patient also describes a new "pins and needles" sensation in both of his feet. His medical history is notable for type 2 diabetes mellitus, essential hypertension, and hyperlipidemia. Current medications include metformin, lisinopril, and atorvastatin. Vital signs are within normal limits. Physical examination reveals a wide-based, unsteady gait and decreased vibratory sensation in the bilateral toes and ankles.

Laboratory studies reveal the following:

Laboratory Test	Result	Reference Range
Hemoglobin	$10.4$ g/dL	$13.5-17.5$ g/dL
Mean corpuscular volume	$112$ fL	$80-100$ fL
Serum vitamin $B_{12}$	$260$ pg/mL	$200-900$ pg/mL
Serum folate	$14.2$ ng/mL	$>4.0$ ng/mL

Which of the following is the most appropriate next step in evaluation?

Measurement of serum methylmalonic acidCevap
B
Measurement of anti-intrinsic factor antibodies
C
Magnetic resonance imaging of the brain and spinal cord
D
Measurement of hemoglobin A1c
E
Measurement of serum ferritin and total iron-binding capacity

Cevap

Measurement of serum methylmalonic acid

The correct answer is the measurement of serum methylmalonic acid (MMA). This patient presents with classic signs of vitamin B12 (cobalamin) deficiency, including macrocytic anemia, peripheral neuropathy (diminished vibratory sensation, ataxia), and cognitive decline. He is also at increased risk due to his chronic use of metformin, which can impair the ileal absorption of vitamin B12. His initial serum vitamin B12 level is borderline (typically

200-300

pg/mL). Because serum B12 assays can have variable sensitivity and specificity, patients with strong clinical presentations but borderline serum levels require confirmatory testing. Vitamin B12 is an essential cofactor for the conversion of methylmalonyl-CoA to succinyl-CoA. Therefore, measuring serum MMA, which will be elevated in a true deficiency state, is the most appropriate next step in laboratory evaluation.

Adım Adım Çözüm

Analyze the patient's clinical presentation and medical history.

The patient has progressive cognitive decline, peripheral neuropathy, and ataxia, with a history of metformin use (a known risk factor for B12 malabsorption).

To establish a clinical suspicion for the underlying cause of the new neurologic symptoms.

Interpret the initial laboratory findings.

The complete blood count shows a macrocytic anemia (MCV

112

fL). The vitamin

B_{12}

level is borderline (

260

pg/mL) and folate is normal.

To correlate the clinical findings with objective biochemical data.

Determine the appropriate diagnostic test to confirm the suspected diagnosis.

Measurement of serum methylmalonic acid is selected to confirm functional tissue deficiency of vitamin B12.

When serum vitamin B12 levels are borderline, a sensitive and specific confirmatory test is required before initiating therapy or etiologic workup.

Anahtar Kavram

Diagnosing vitamin B12 deficiency in patients with strong clinical suspicion but borderline serum levels using methylmalonic acid.

Soru

Cevap

Adım Adım Çözüm

Anahtar Kavram

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