If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 2 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.
This month’s stumper
A 35-year-old man complains of chronic vague gastric pain of several years’ duration. The pain is sometimes relieved by food. Serum immunoglobulin studies for IgG and IgA antibodies directed against Helicobacter pylori are strongly positive.
Endoscopy with gastric antral biopsy demonstrates gastritis but no ulcerative lesions. H. pylori organisms are seen with special stains on the biopsy fragments. The patient is treated with a one-week course of omeprazole (20 mg bid), plus clarithromycin and metronidazole (500 mg bid each).
Which of the following is the most appropriate noninvasive test to determine whether the H. pylori has been eradicated?
A. Culture of gastric biopsy.
B. Rapid urease test.
C. Repeat qualitative lgA and lgG antibodies against H. pylori.
D. Repeat quantitative lgA and lgG antibodies against H. pylori.
E. Urea breath test.
The correct answer is E.
Kaplan Medical explains why
Helicobacter pylori is a small, gram-negative bacterium that lives in and locally destroys the mucus coating that lines the stomach. The organism has been linked to a wide variety of problems, including gastritis, peptic ulcer disease, gastric cancer, and gastric lymphoma. Because of these associations, physicians have become more aggressive about therapy.
H. pylori is a hardy organism and requires concurrent therapy with multiple agents for eradication. The original regimen combined bismuth subsalicylate, tetracycline and metronidazole, and had only an 80 percent cure rate in compliant patients who followed the regimen for two weeks. The schedule listed in the question stem is a more modern one; it is both easier to follow and has a better cure rate.
Alternative regimens may substitute amoxicillin (1 g bid) for metronidazole, or may substitute lansoprazole (30 mg bid) for omeprazole. These more effective regimens have caused a problem in determining whether eradication has occurred, however, because the course is so short that IgG and IgA antibodies against H. pylori have not had time to decrease by the end of therapy.
The urea breath test is a relatively new test in which the patient is given oral urea that has been labeled with 13C or 14C. The H. pylori bacteria contain the enzyme urease and are able to metabolize the urea, producing radioactively labeled CO2, which can be measured in breath samples taken 20–30 minutes after ingestion. It is recommended that this test be delayed until four weeks after the end of the regimen, since recent antibiotic use may have decreased the number of organisms enough to produce a negative test, without having achieved true eradication.
Why the other answers are wrong
Choice A: Culture of gastric biopsy is highly specific but requires both endoscopic biopsy and fastidious culture technique. Therefore, this method is not often used clinically for follow-up studies.
Choice B: The rapid urease test is performed on gastric tissue. It is rapid, specific, and sensitive, but requires endoscopy to obtain the biopsy fragment.
Choice C: Qualitative assays of antibodies against H. pylori may be positive for up to three years after eradication of the infection.
Choice D: Quantitative assays of antibodies against H. pylori drop slowly for up to a three years after eradication of the infection.
For more prep questions on USMLE Steps 1, 2 and 3, view other posts in this series.
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