Heat oven to 400 degrees F. Grease bottoms of twelve 2-1/2-inch muffin-pan cups.
Stir together the flour, sugar, baking powder and salt in a large bowl.
Lightly beat the egg in a small bowl. Beat in the milk, melted butter, lemon rind and lemon juice until well blended.
Pour the egg mixture all at once into the flour mixture; mix lightly with fork, stirring until dry ingredients are just moistened (batter will be lumpy). Fill each prepared muffin cup about two-thirds full.
Bake in 400 degree F oven for 25 minutes or until a wooden pick inserted in centers comes out clean. Remove the muffin pan to a wire rack. Gently loosen muffins in pan by running a thin metal spatula around the side of each muffin cup. Let muffins cool in pan 5 minutes. Turn the muffins out onto the wire rack. Serve warm or cool completely to serve later. Makes 12 muffins.