USMLE® Step 1 style questions USMLE
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A 15-year-old teenage boy comes to the office for an evaluation of gross hematuria. His family history is significant for the death of 2 siblings due to renal failure. Upon medical interrogation, the patient reports that lately, he has noticed blurry vision while looking at objects far away. On physical exam, the patient looks well-appearing. Assessment of his visual acuity shows 20/40. A complete eye exam reveals a right corneal erosion and multiple lens opacities on both eyes. His temperature is 36.7°C (98.2°F), pulse is 61/min, respirations are 16/min, blood pressure is 110/60 mmHg. A kidney biopsy shows thinning of the glomerular basement membrane, in addition to, the immunochemistry staining absence of alpha-5-chain of type 4 collagen (COL4A5) proteins. At this point, which of the following findings is most likely to occur in this patient as a result of his condition?
Each member of the family is named with a Roman numeral, and if mutated or absent, can lead to problems in the tissues where that particular collagen is found.
Alport syndrome occurs as a result of mutations in Type IV collagen, which is particularly important in the glomerulus of the kidney, the eye, and the cochlea, and that’s why the symptoms of Alport syndrome are specific to those tissues.
Type IV collagen is a sheet-like structure found in all basement membranes and serves to support cells and form barrier.
The three basement membrane layers are the lamina lucida, lamina densa (where type IV collagen is), and lamina reticularis.
Now within the kidneys, there are glomeruli, which filter the blood and that together with a tubule forms a nephron.
These glomeruli happen to have a basement membranes, called the glomerular basement membrane, or GBM, and that GBM, along with the fenestrated, meaning has pores, capillary endothelium and the podocyte slit diaphragm, forms a selective filter, meaning that water and certain other plasma components can escape the capillary, forming the filtrate that will become urine, but red blood cells and most proteins stay in the glomerular capillary.
In Alport syndrome, kidney function is normal through early childhood, but over time, the missing or nonfunctional type IV collagen causes the GBM to become thin and overly porous.
This allows red blood cells to pass right through from the capillary to the urinary filtrate leading to microscopic hematuria, which is where red blood cells are seen in the urine under a microscope, and this might eventually lead to gross hematuria, where the red blood cells can be seen with the naked eye.
And finally this excessive protein loss and other factors cause the GBM to undergo sclerosis.
As there are less and less healthy glomeruli, somebody with Alport syndrome might develop renal insufficiency or even failure, which can lead to renovascular hypertension as well.
Together the hematuria, renal insufficiency, and hypertension contribute to the categorization of Alport syndrome as a glomerulonephritis.
So although abnormality in the basement membrane is from a different cause, the result is similar in these two syndromes.
Another part of the body affected in Alport syndrome is the ear, which can lead to hearing loss, and usually this isn’t present at birth, but starts in childhood or adolescence, and the precise mechanism is unclear.
One proposed mechanism involves the cochlea or inner ear structure which contains the Organ of Corti, which has really small, but highly specialized set of hair cells that are attached to a basement membrane.
Abnormal type IV collagen might prevent those hair cells from generating normal nerve signals in response to sound vibrations.
Some patients with Alport syndrome also have eye findings, which are also not typically present at birth.
Myopia, or nearsightedness, and lens opacities can result as well.
Other eye changes in Alport syndrome include having white or yellow flecks around the macula, which is the central part of the retina, as well as having problems with the cornea, like recurrent erosions.