USMLE PRACTICE QUESTION #121
TOPIC: Hematology/Pathology
USMLE practice question #121 will test your understanding of a commonly tested hematology concept.
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A 62-year-old woman presents to her physician with complaints of fatigue, weakness, and a sensation of "pins and needles" in her hands and feet. She also reports difficulty with balance and occasional memory problems. Physical examination reveals a smooth, beefy red tongue and decreased vibratory sensation in her lower extremities. Laboratory studies show a macrocytic anemia with a low serum vitamin B12 level. A Schilling test is performed and demonstrates impaired vitamin B12 absorption that is not corrected with the administration of intrinsic factor. Which of the following is the most likely underlying cause of this patient's condition?
A) Pancreatic insufficiency
B) Ileal resection due to Crohn's disease
C) Infection with Helicobacter pylori
D) Chronic atrophic gastritis with loss of parietal cells
E) Dietary deficiency of vitamin B12
Detailed Breakdown of Answers + Correct Answer Below ⏬
ANSWER + QUESTION BREAKDOWN
It’s important to adopt the correct MENTAL MODEL when answering USMLE questions; it saves time and increases accuracy. The mental model outlined below is a foundational component of our test-taking skills masterclass (check it out if you want to elevate your skills). Here’s how to think through this question:
Step 1. Read the last line to get to the heart of the question: “Which of the following is the most likely underlying cause of this patient's condition?”
Step 2: Is this a first-, second-, or third-order question?
Answer: 2nd order. 1st: Determine the meaning of the Schilling test results; 2nd: Identify the most likely cause of the patient’s symptoms.
Step 3: Read the vignette carefully and ask yourself: “Based on the given test results, the most likely cause of this patient’s symptoms is _____________”
Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.
GENERAL ANALYSIS
This patient has vitamin B12 deficiency, as evidenced by her macrocytic anemia, neurological symptoms (paresthesia and balance issues), and low serum B12 levels. The Schilling test indicates impaired absorption of vitamin B12 that is not corrected by intrinsic factor administration, which rules out pernicious anemia (caused by intrinsic factor deficiency).
ANSWER CHOICES:
CHOICE A: Pancreatic insufficiency
Explanation: Pancreatic enzymes are needed to release vitamin B12 from haptocorrin in the duodenum, but this defect would be corrected by intrinsic factor administration in the Schilling test. Thus, it is not consistent with this case.
CHOICE B: Ileal resection due to Crohn's disease
Explanation: Ileal resection, often due to Crohn's disease or other conditions, is a common cause of impaired vitamin B12 absorption. The terminal ileum contains specialized receptors for the intrinsic factor-vitamin B12 complex. If this portion of the intestine is surgically removed or severely inflamed (as in Crohn's disease), vitamin B12 absorption is compromised, leading to deficiency.
CHOICE C: Infection with Helicobacter pylori
Explanation: H. pylori infection can cause chronic gastritis, but it does not directly impair vitamin B12 absorption in the ileum or lead to the findings on the Schilling test.
CHOICE D: Chronic atrophic gastritis with loss of parietal cells
Explanation: This condition leads to intrinsic factor deficiency and pernicious anemia. However, the Schilling test shows that administering intrinsic factor does not correct the absorption defect, ruling out this cause.
CHOICE E: Dietary deficiency of vitamin B12
Explanation: Dietary B12 deficiency is rare except in strict vegans or those with severe malnutrition. It would not explain the Schilling test results showing impaired absorption uncorrected by intrinsic factor.
FINAL VERDICT…
CORRECT ANSWER: B) Ileal resection due to Crohn's disease
This patient has vitamin B12 deficiency, as evidenced by her macrocytic anemia, neurological symptoms (paresthesia and balance issues), and low serum B12 levels. The Schilling test indicates impaired absorption of vitamin B12 that is not corrected by intrinsic factor administration, which rules out pernicious anemia (caused by intrinsic factor deficiency). This result strongly suggests a problem with the terminal ileum, where vitamin B12 is absorbed.
Ileal resection, often due to Crohn's disease or other conditions, is a common cause of impaired vitamin B12 absorption. The terminal ileum contains specialized receptors for the intrinsic factor-vitamin B12 complex. If this portion of the intestine is surgically removed or severely inflamed (as in Crohn's disease), vitamin B12 absorption is compromised, leading to deficiency.
KEY CONCEPTS:
Vitamin B12 Absorption Pathway:
Vitamin B12 binds to intrinsic factor (produced by gastric parietal cells) in the stomach.
The intrinsic factor-B12 complex is absorbed in the terminal ileum via specific receptors.
Schilling Test:
A diagnostic test to determine the cause of vitamin B12 deficiency.
Part 1: Measures baseline absorption of radiolabeled vitamin B12.
Part 2: Measures absorption after administering intrinsic factor.
If absorption improves with intrinsic factor, it indicates a deficiency of intrinsic factor (e.g., pernicious anemia).
If absorption does not improve with intrinsic factor, it suggests a problem with the terminal ileum or other causes of malabsorption.
Causes of Vitamin B12 Deficiency:
Intrinsic Factor Deficiency:
Conditions like pernicious anemia or chronic atrophic gastritis (loss of parietal cells).
Corrected by intrinsic factor in Schilling test.
Terminal Ileum Dysfunction:
Causes include Crohn’s disease, ileal resection, or other ileal damage.
Not corrected by intrinsic factor in Schilling test.
Other Causes:
Pancreatic insufficiency (affects release of B12 from binding proteins).
Dietary deficiency (rare except in strict vegans or malnutrition).
Interpretation of This Case:
The patient’s Schilling test showed no improvement with intrinsic factor, ruling out intrinsic factor deficiency (e.g., pernicious anemia).
This strongly suggests terminal ileum dysfunction, such as ileal resection due to Crohn’s disease.
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