If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 1 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
An investigator is examining the distribution of an ion channel protein in the kidney. Slices of kidney tissue are incubated in a dilute solution of a specific antibody directed against the protein. The immunoperoxidase method is then used to localize the ion channel proteins. In one area, the investigator notes epithelial cells with a brush border that are positive for the ion channel protein.
Which of the following areas is most likely to show these microscopic characteristics?
A. Collecting duct.
B. Descending thin limb of the loop of Henle.
C. Distal convoluted tubule.
E. Proximal convoluted tubule.
The correct answer is E.
Kaplan Medical explains why
The proximal convoluted tubule (PCT) is the only portion of the renal tubule in which the epithelial cells have a "brush border." The brush border is composed of microvilli, which greatly increases apical membrane surface area and thereby enhances epithelial reabsorptive capacity. The PCT recovers almost 100% of filtered organic solutes (e.g., glucose, amino acids, proteins) and about 67% of electrolytes and water, amounting to about 120 L of the daily filtered load.
Why the other answers are wrong
Choice A: The collecting ducts are lined by regular simple cuboidal epithelial cells, and they do not have a brush border. The collecting duct system reabsorbs water primarily, amounting to about 18% of the daily filtered load.
Choice B: The descending thin limb of the loop of Henle has a simple squamous epithelium lining without a brush border. Water diffuses freely between tubule lumen and interstitium in response to osmotic gradients.
Choice C: The distal convoluted tubule is lined by cuboidal epithelial cells without a brush border. The tubule is capable of establishing steep ionic concentration gradients (i.e., a tight epithelium with low water permeability) and is an important site for regulated reabsorption of Mg2+ and Ca2+.
Choice D: The glomerulus is composed of capillary loops interposed between two (afferent and efferent) arterioles. The capillary walls are composed of simple squamous epithelial cells and are fenestrated, designed to deliver about 180 L of pressurized plasma ultrafiltrate daily into the renal tubule.
Tips to remember
- The PCT epithelium has a microvillar "brush border" that enhances reabsorptive surface area.
- No other tubule segments contain a brush border.
For more prep questions on USMLE Steps 1, 2 and 3, view other posts in this series.
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