ABIM Exam Review Practice Question of the Week: 42 y.o. Dentist with Fatigue, Paresthesias, and Abnormal Gait

42-year-old female presents to the clinic for fatigue and paresthesias of her lower extremities. She has noticed that she has had an abnormal gait. These problems all occurred within the past few days. She has no other medical problems or surgical history. She denies smoking, alcohol, or illegal drugs. She is a dentist and feels she has a balanced, unrestricted diet. She eats meat, fruits, vegetables and carbohydrates and avoids fatty and oily foods. Family history is non-contributory. Her physical exam is positive for pallor, loss of vibratory sense in both lower extremities, and also loss of conscious proprioception. CT of the head is negative for any acute process. Labs show the following:


  • Hemoglobin 8.2g/dL
  • MCV 104fl
  • Vitamin B12 100pg/mL
  • Folate 10ng/mL
  • Reticulocyte count 0.3%


Which of the following describes the likely cause of her condition?


A. Alcohol abuse


B. Autoimmune disorder


C. Environmental exposure


D. Bacterial overgrowth


E. Poor diet





This patient has developed her symptoms in a matter of days. She has elevated mean corpuscular volume (MCV), anemia, low vitamin B12 level, and neurological symptoms that point towards vitamin B12 deficiency.


Let’s go through the answer choices.


● Choice A (Alcohol abuse) would cause folate deficiency. Folate deficiency will also show an elevated MCV but it will not present with paresthesias in the lower extremities, as seen in this patient.


● Choice B (Autoimmune disorder), with regards to causes of anemia, refers to pernicious anemia. With this diagnosis, antibodies against intrinsic factor prevent vitamin B12 absorption. This patient doesn’t likely have this condition because this will be a more chronic process than the acute presentation she is experiencing.


● Choice C (Environmental exposure) is the correct answer. This patient is a dentist and is exposed to nitrous oxide or laughing gas. Nitrous oxide can inactivate vitamin B12 very quickly and cause anemia and neurological symptoms to occur in a rapid manner.


● Choice D (Bacterial overgrowth) is incorrect because the patient would complain of foul-smelling diarrhea. Patients with bacterial overgrowth will likely also have increased folate levels and decreased vitamin B12 levels.


● Choice E (Poor diet) is incorrect also because it is mentioned in her social history that she consumes a balanced assortment of foods. Individuals who are on a vegan diet are at increased risk of developing vitamin B12 deficiency.


You can see all the previous ABIM Exam Review Questions of the Week at the Knowmedge Blog. You can find also additional topics and questions directly from the Knowmedge Internal Medicine ABIM Board Exam Review Questions QVault.

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