。
It begins pulsing again inside the patient's body.
Literal
Patient [possessive-の] body-inside [at-で] again pulse-starts.
Dramatic and terse — likely describing a heart or transplanted organ resuming function. 脈打つ ('to pulsate, to throb') combines 脈 ('pulse') with 打つ ('to strike'). The subject is entirely omitted, forcing the reader to supply it from context — a technique that heightens suspense in narration.