Whisk egg, mayonnaise, mustard, pepper, salt and cayenne in a medium bowl until combined. Stir in scallions. Add crab and panko; stir to combine. Form the mixture into four 1/2-inch-thick patties.
Heat oil in a large nonstick skillet over medium-high heat until shimmering. Add the crab cakes and cook, turning once, until golden brown, 4 to 5 minutes per side. Serve with lemon wedges.
2 1/2 fat, 2 1/2 lean protein, 1/2 starch