The ornamental design for a medicalinstrument, as shown and described.
FIG. 1 is a front view of a medical instrument showing my new design;
FIG. 2 is a side view thereof;
FIG. 3 is a perspective view thereof;
FIG. 4 is an exploded perspective view thereof;
FIG. 5 is a bottom view thereof; and,
FIG. 6 is a top view thereof.
The broken lines illustrate portions of the medical device that form no part of the claimed design; the broken lines and the unshaded surfaces form no part of the claimed design.