Which component of the plasma membrane contains a hydrophobic region and acts as the primary barrier to most foreign substances?

The component of the plasma membrane that contains a hydrophobic region and acts as the primary barrier to most foreign substances is the phospholipid bilayer. The bilayer consists of phospholipid molecules with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward, creating a barrier that restricts the passage of polar and charged substances. This hydrophobic core effectively prevents many foreign substances from easily entering or exiting the cell.