What led to A.J. Puk’s struggles in his fourth spring outing

Oakland Athletics pitcher A.J. Puk throws in the bullpen at the Lew Wolff Training Complex during Spring Training workouts for the A’s on Sunday, Feb. 18, 2018, in Mesa, Ariz. (Aric Crabb/Bay Area News Group)MESA, Ariz. — It appears A’s top pitching prospect A.J. Puk is human after all.

After allowing no earned runs through his first three appearances of the Cactus League, Puk lasted just 2 2/3 innings as he was touched up for four runs on three hits, three walks, and a wild pitch in Thursday’s 6-2 loss to the Mariners. He registered just one strikeout on the night.

While Puk’s fastball reached 97 mph at one point this spring, his fastball velocity dropped to about 90-91 mph after the first…