USMLE® Step 1 style questions USMLE
A 40-year-old woman comes to the clinic for a follow-up appointment after hospitalization for pyelonephritis. The patient has had multiple episodes of urinary tract infections over the past few years. She is otherwise healthy. She does not use tobacco, alcohol or illicit substances. Family history is unremarkable. The patient’s physical exam is normal. A CT of the abdomen obtained during hospitalization is shown below:
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Which of the following best explains these findings?
Content Reviewers:Rishi Desai, MD, MPH
Contributors:Tanner Marshall, MS
Horseshoe kidney, or sometimes renal fusion, is a congenital disorder where the two kidneys fuse together into one during fetal development, resulting in one large horseshoe-shaped kidney.
Normally during fetal development, the future kidneys develop in the pelvis as a structure called the metanephrose before finally migrating upward into the abdomen and becoming everyone’s favorite organ duo.
For people with horseshoe kidney, their kidneys fused together at some point during development, and there are two main working theories on how this might happen.
The first is mechanical fusion, which happens during the metanephros stage, which is around about the 5th week of gestation.
At this point, the two kidneys are still in the pelvis, and are therefore pretty close together, so close that it’s thought that some flexion or growth of the developing spine and pelvic organs essentially pushes them together, causing the lower or inferior poles of the kidneys to touch and fuse together, forming what’s called a fibrous isthmus, fibrous because it’s composed of connective tissue.
In this case it’s thought that the posterior nephrogenic cells, which are the cells that help to form part of the kidney, migrate and rendezvous in the wrong spot, and therefore again form an isthmus connecting the two kidneys, but this time since the isthmus is composed of kidney cells as opposed to connective tissue, it’s called a parenchymal isthmus.
Whichever one happens, now you’ve got this single, horseshoe-shaped kidney in the pelvis.
- "Robbins Basic Pathology" Elsevier (2017)
- "Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2)" McGraw-Hill Education / Medical (2018)
- "Pathophysiology of Disease: An Introduction to Clinical Medicine 8E" McGraw-Hill Education / Medical (2018)
- "CURRENT Medical Diagnosis and Treatment 2020" McGraw-Hill Education / Medical (2019)
- "HORSESHOE KIDNEY" Annals of Surgery (1925)
- "Multimodality imaging spectrum of complications of horseshoe kidney" Indian Journal of Radiology and Imaging (2017)
- "Horseshoe kidney: a review of anatomy and pathology" Surgical and Radiologic Anatomy (2013)