Full Tilt No-Limit Hold'em, $2 BB (9 handed) from (Format: FlopTurnRiver)

saw flop|saw showdown

UTG ($110.85)
UTG+1 ($194)
MP1 ($134.95)
MP2 ($104.80)
MP3 ($198)
CO ($60)
Hero ($293.55)
SB ($294.70)
BB ($40)

Preflop: Hero is Button with 2, 2.
4 folds, MP3 calls $2, 1 fold, Hero raises to $9, 2 folds, MP3 calls $7.

Flop: ($21) 4, K, 2 (2 players)
MP3 checks, Hero bets $14, MP3 calls $14.

Turn: ($49) A (2 players)
MP3 bets $25, Hero raises to $75, MP3 calls $50.

River: ($199) K (2 players)
MP3 bets $100 (All-In), Hero ???

OK. So take a look at this hand. All you pros out there, what's your thought process on the this river raise?