How to solve USMLE Clinical Vignettes-Renal Physiology Case 1

USMLE Clinical Vignette Physio/Renal

A 4-year-old boy is brought to the physician because of slow growth during the past year. He has had recurrent urinary tract infections since the age of 1 year. He is at the 10th percentile for height and 25th percentile for weight. Physical examination shows pallor. Laboratory studies show a normochromic, normocytic anemia and increased serum concentrations of urea nitrogen and creatinine. Urinalysis shows a low specific gravity. Which of the following sets of additional serum findings is most likely in this patient?

Calcium Inorganic Phosphorus 1,25-Dihydroxycholecalciferol Erythropoietin

(A) ↑ ↑ ↑ ↓

(B) ↑ ↑ ↓ ↓

(C) ↑ ↓ ↓ ↑

(D) ↓ ↑ ↑ ↓

(E) ↓ ↑ ↓ ↓

(F) ↓ ↓ ↑ ↑

When you go through the above clinical vignette, it describes a 4 y/o boy with slow growth and renal failure( high BUN and Creatinine). When you are trying to solve a vignette with multiple confusing options with up and down arrows, first pick a parameter you are highly confident of…i think as renal failure is obvious, for most people it is easy to remember the vital role of kidneys in Vit D ( 1,25-Dihydroxycholecalciferol) synthesis. So you can confidently expect Vit D levels to be low in Renal Failure. That brings us to options B,C and E. Also, many of you are well versed with kidney and erythropoieitin, expect levels to be low also ( just a reminder, EPO levels will rise in Renal Carcinoma though). So with the above two information, we can now cut down to options B and E. Good thing..we just have to choose between high or low calcium…so if you now remember that Vit D is low in this case, calcium absorption will be affected and levels will decrease. Also it is worthwhile to know that low calcium is going to trigger the parathyroid gland to release more PTH causing Secondary Hyperparathyroidism (PTH will be high). So, the correct answer is (E).

Good luck, contact me if you have any questions.

Dr. Sujata Arecanteparamb

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