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?