USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 20-year-old man comes to the office because he has been getting short of breath during his usual exercise routine. He reports seeing blood in his urine a few times in the last month, and has noticed that his eyes tend to swell for no apparent reason. His serum creatinine is 2.5 mg/dl and blood urea nitrogen level is 26 mg/dl. Which of the following would most likely be seen on histologic examination of a kidney biopsy?
Goodpasture syndrome is an autoimmune disease that primarily affects two organs - the lungs and the kidneys.
It causes inflammation and eventually bleeding in the lungs which leads to ‘hemoptysis’ or coughing up of blood, and hematuria or blood in the urine, a pattern first recognized by the pathologist - Dr. Ernest Goodpasture.
To understand Goodpasture syndrome, let’s start by thinking about the basement membrane which is a thin, sheet-like layer of tissue made of protein that keeps the epithelium stuck firmly to actual organ - a bit like double-sided tape which keeps gift wrapping paper stuck to the gift.
The basement membrane is made up of various proteins, but the major one is collagen, and since basement membrane exists throughout every organ system, it’s no wonder that collagen is the most abundant protein in the human body.
As far as proteins go, collagen is a pretty awesome looking one, with a triple-helix structure composed of three separate chains that are intertwined like braided hair.
Each of the chains can be one of six types, named α1 through α6, and the most common form of collagen found in the basement membrane is collagen type IV, which is made by mixing and matching these six α-chains.
One version of type IV collagen combines the α3, α4, and α5 chains. Another combines two α1’s and an α2. A third version has two α5’s and an α6. And so on.
So it turns out that the α3/α4/α5 variant is most common in the glomerular basement membrane of the kidneys and the alveolar basement membrane of the lungs.
In Goodpasture syndrome, autoantibodies bind to a specific part of the α3 chain that is usually hidden deep within the folded chains.
This is an example of a type II hypersensitivity reaction, because once these autoantibodies, usually IgG but rarely IgM or IgA, bind to the the α3 chain, they activate the complement system. The complement system is a series of small proteins present in the blood that act like an enzymatic cascade to fight off bacterial and other pathogenic invasions.
When the Fab portion of the IgG molecule inappropriately binds to the α3 chain, C1, the first of the complement proteins, binds to the Fc portion of the IgG. This bound C1 is now activated and it starts engaging other members of the complement family, C2 through C9.
Some of these are activated by being cleaved or chopped by an enzyme. The cleaved fragments C3a, C4a, and C5a act as chemotactic agents meaning they attract certain cells like neutrophils.
Once neutrophils join the party, they dump a bunch of enzymes like peroxidase, myeloperoxidase, and proteinase-3 which all cause free oxygen radicals to form which damage the basement membrane as well as the nearby endothelium and the underlying organ itself.
Genetic risk factors for Goodpasture syndrome include having genes that encode a specific type of immune molecule called HLA-DR15 which is used to identify and bind to foreign molecules.