A 4-year-old girl has had puffy eyelids each morning for two weeks and now has swollen legs and a distended abdomen. Her blood pressure is 96/58 mmHg. Urine testing shows protein 4+ and no blood, and microscopy shows no red cells or casts. Her urinary protein is almost entirely albumin. Plasma albumin is 16 g/L and creatinine 32 micromol/L.
A defect in which one of the following components of the renal corpuscle best accounts for this girl's urinary findings?