A mutation in the enzyme lactase results in the substitution of alanine with threonine at a crucial active site, leading to a significant decrease in the rate of lactose digestion. Explain why this single amino acid substitution causes such a drastic change in the function of lactase.

Threonine decreases the size of the enzyme, reducing the enzyme's overall surface area for lactose binding.

Threonine has a larger side chain and introduces a polar hydroxyl group, which disrupts the active site's ability to bind lactose.

Alanine and threonine are both nonpolar amino acids, so the substitution should not affect the enzyme's function.

Threonine is unable to form covalent bonds with lactose, preventing the enzyme from breaking down the sugar.