A 40-year-old man with Alport syndrome presents with a 3-month history of headaches. His blood pressure is 165/100 mm Hg. A urinalysis shows 3+ proteinuria and 2+ hematuria. Laboratory studies disclose elevated levels of BUN (48 mg/dL) and creatinine (3.6 mg/dL). This patient's renal disease is caused by mutation in a gene that encodes which of the following extracellular matrix proteins?

Hereditary nephritis (Alport syndrome) reflects abnormal type IV collagen in the glomerular basement membrane. The syndrome is a proliferative and sclerosing glomerular disease, often accompanied by defects of the ear or the eyes, which is caused by a genetic abnormality in type IV collagen. Hematuria is present early in life in males with X-linked disease and in both sexes with autosomal recessive disease. Proteinuria, progressive renal failure, and hypertension develop later in the course of the disease. Virtually all men with the X-linked syndrome and both sexes with autosomal recessive disease develop end-stage renal disease by ages 40 to 50 years. Patients with Marfan syndrome have mutations in the fibrillin gene (choice C).Diagnosis: Hereditary nephritis, Alport syndrome