A 68-year-old man presents to the clinic with a 3-month history of progressive fatigue, ex… — USMLE Step 2 CK

A $68$ -year-old man presents to the clinic with a $3$ -month history of progressive fatigue, exertional shortness of breath, and a tingling sensation in both feet. He has a history of hypothyroidism and type $2$ diabetes mellitus. His medications include levothyroxine and metformin. Physical examination reveals pale conjunctivae, a smooth, red tongue, and decreased vibratory sensation in the lower extremities bilaterally.

Laboratory studies reveal the following:

Test	Result	Reference Range
Hemoglobin	$8.2 \text{ g/dL}$	$13.5 - 17.5 \text{ g/dL}$
Mean corpuscular volume (MCV)	$116 \text{ fL}$	$80 - 100 \text{ fL}$
Platelets	$140,000 \text{ /mm}^3$	$150,000 - 400,000 \text{ /mm}^3$
Leukocytes	$4,200 \text{ /mm}^3$	$4,500 - 11,000 \text{ /mm}^3$

A peripheral blood smear demonstrates macro-ovalocytes and multiple neutrophils with six or more nuclear lobes. Due to a misinterpretation of the preliminary results, the patient is exclusively prescribed high-dose oral folic acid.

Which of the following is the most likely outcome of this inappropriate monotherapy?

Partial or complete reversal of the megaloblastic anemia with progression of the neurologic deficitsAnswer
B
Resolution of both the hematologic and neurologic abnormalities
C
Improvement of the neurologic symptoms with persistent macrocytic anemia
D
Development of new-onset severe, life-threatening hypokalemia
E
Precipitation of an acute intravascular hemolytic crisis

Answer

The correct answer is the option stating that there will be partial or complete reversal of the megaloblastic anemia with progression of the neurologic deficits.

The patient's presentation of fatigue, macrocytic anemia (MCV > 100 fL), hypersegmented neutrophils, and glossitis paired with neurologic deficits (decreased vibratory sensation, tingling) is the hallmark of Vitamin B12 (cobalamin) deficiency. The patient's long-term use of metformin is a known risk factor for B12 malabsorption. Vitamin B12 and folate are both required for DNA synthesis. In B12 deficiency, folate becomes trapped as 5-methyltetrahydrofolate, leading to impaired DNA synthesis and megaloblastic anemia. Administering high-dose folic acid bypasses this 'folate trap', providing the necessary substrates to resume DNA synthesis, thereby correcting the hematologic abnormalities. However, Vitamin B12 is independently required as a cofactor for the enzyme methylmalonyl-CoA mutase, which is critical for proper myelin synthesis in the nervous system. Folic acid has no effect on this pathway. Therefore, inappropriate folic acid monotherapy will correct the anemia (masking the most obvious sign of the disease) while allowing irreversible demyelination and neurologic deficits to progress unchecked.

Step-by-Step Solution

Diagnose the underlying condition based on the clinical presentation and laboratory findings.

The patient has macrocytic anemia (MCV 116 fL), hypersegmented neutrophils, glossitis, and peripheral neuropathy (decreased vibratory sensation). This classic constellation, combined with chronic metformin use, points to Vitamin B12 (cobalamin) deficiency.

Identifying the true deficiency is required to understand the consequences of the incorrect treatment.

Analyze the biochemical relationship between Vitamin B12 and folic acid in DNA synthesis.

Vitamin B12 is required to transfer a methyl group from 5-methyltetrahydrofolate (5-methyl THF) to homocysteine. Without B12, folate is trapped in the 5-methyl THF form (the 'folate trap'), halting DNA synthesis and causing megaloblastic anemia.

Understanding this pathway explains why high doses of folic acid can 'bypass' the trap and temporarily fix DNA synthesis.

Determine the impact of folic acid monotherapy on the neurologic symptoms of B12 deficiency.

Neurologic damage in B12 deficiency is due to impaired conversion of methylmalonyl-CoA to succinyl-CoA, leading to abnormal myelin accumulation. Folic acid plays no role in this pathway.

This confirms that while folate fixes the blood count, the spinal cord and peripheral nerve damage will quietly worsen.

Key Concept

Administration of high-dose folic acid to a patient with Vitamin B12 deficiency will correct the megaloblastic anemia but will mask the deficiency, allowing irreversible neurologic degeneration to progress.

Estimated Time:1m 15s

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