I am going to answer the sample USMLE Step 1 Questions with Explanation. These questions are available at USMLE.org
A 5-year-old girl is brought to the emergency department because of a 2-day history of fever, urinary urgency, and burning pain with urination. She has had four similar episodes during the past year. A diagnosis of urinary tract infection is made. Subsequent renal ultrasonography shows one large U-shaped kidney.
Which of the following is the most likely embryologic origin of this patient’s condition?
(A) Failure of the kidneys to rotate 90 degrees medially
(B) Failure of normal kidney ascent
(C) Failure of one ureteric bud to develop normally
(D) Fusion of the inferior poles of the metanephros during ascent
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During embryonic development, the kidneys originate from the intermediate mesoderm and undergo a complex series of events to achieve their final position and structure. The metanephros, which is the definitive kidney, initially forms as a pair of buds that ascend from the pelvis to the posterior abdominal wall.
In this scenario, fusion of the inferior poles of the metanephros during ascent can lead to the formation of a single large kidney with a U-shaped appearance. This fusion typically occurs due to abnormal development or migration of the kidneys during the ascent process, resulting in a single large kidney with an abnormal shape.
Hydronephrosis, stone formation, infection, and renal cell carcinoma are higher in these patients, resulting in a diseased horseshoe kidney.
The correct answer is (D) Fusion of the inferior poles of the metanephros during ascent