USMLE-Rx Step 1 Practice Q's

USMLE-Rx Step 1 Qmax Challenge #1161

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USMLE-Rx Step 1 Qmax Challenge #1161A 60-year-old woman with rheumatoid arthritis presents to her doctor with a 2-month history of progressive fatigue. She also reports feeling “wobbly” lately. Physical examination reveals a pulse of 101/min and decreased light touch and vibration sense on her lower extremities. Laboratory studies show a hemoglobin level of 9 g/dL and a mean corpuscular volume of 110 fL. A peripheral blood smear for the patient is shown in the image. On further analysis, methylmalonic acid levels and homocysteine levels are elevated.

This patient’s anemia most likely results from which of the following factors?

A. A vitamin deficiency resulting from medication administered for her rheumatoid arthritis
B. Abnormal neural crest cell migration
C. Antibodies against parietal cells
D. Bacterial overgrowth in the colon
E. Partially occlusive embolus to the superior mesenteric artery


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11 replies »

  1. The correct answer is C. This patient has neurologic symptoms consistent with vitamin B12 (cobalamin) deficiency, which gradually causes demyelination of the dorsal columns, spinocerebellar tract, and lateral corticospinal tract. Pernicious anemia is a common cause of vitamin B12 deficiency and is characterized by chronic atrophic gastritis and autoantibodies directed against gastric parietal cells. As a result, the parietal cells cannot produce normal amounts of intrinsic factor. Without intrinsic factor, vitamin B12 cannot be absorbed in the ileum. Patients may present with symptoms of anemia such as fatigue, dyspnea, and tachycardia. In addition, patients may have symptoms of achlorhydria such as GERD, early satiety, constipation, and weight loss. A peripheral blood smear will show macrocytic RBCs with hypersegmented polymorphonuclear leukocytes, consistent with megaloblastic anemia (as seen in the vignette image). Both methylmalonic acid and homocysteine levels are elevated in B12 deficiency. It is imperative to check folate and vitamin B12 levels before beginning treatment with vitamin B12 injections, because giving the wrong treatment may exacerbate symptoms. Also note that genetic defects such as orotic aciduria can cause a megaloblastic anemia that does not respond to the administration of folate and vitamin B12. Causes of macrocytic anemia that are not megaloblastic include alcoholism, liver disease, reticulocytosis, and medications such as zidovudine.

    A is not correct. Methotrexate is commonly prescribed for rheumatoid arthritis. Its use can cause a folate deficiency. Folate is an essential cofactor in nucleic acid synthesis, and its deficiency commonly leads to megaloblastic anemia (as seen in the image). However, folate replacement should not be prescribed until vitamin B12 deficiency is ruled out, especially in this case, because folate deficiency does not explain this patient’s neurologic symptoms. Homocysteine is elevated in folic acid deficiency, but methylmalonic acid is not.

    B is not correct. Abnormal neural crest cell migration leads to Hirschsprung disease, which is a congenital aganglionic motility disorder affecting the large bowel. Patients present in the first few days of life with obstructive symptoms such as constipation, abdominal distention, and bilious emesis.

    D is not correct. The colon is not the site of vitamin B12 absorption, and bacterial overgrowth there, such as with Clostridium difficile, will produce symptoms including diarrhea, flatulence, and weight loss.

    E is not correct. A fully occlusive embolus to the superior mesenteric artery can lead to an acute bowel infarction, a life-threatening problem. Patients typically present with abdominal pain, bloody stools, fever, and peritoneal signs. Anemia in a patient with acute blood loss is typically a normocytic anemia (normal mean corpuscular volume). For a partially occlusive embolus, noticeable symptoms may not be present if adequate blood flow remains to keep the bowel from becoming ischemic.


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