The ornamental design for medical instrument, as shown and described.
FIG. 1 is a front perspective view of a medical instrument in accordance with the new design;
FIG. 2 is a left side view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a front view thereof; and,
FIG. 7 is a rear view thereof.
The broken lines in FIGS. 2, 4 and 7 are included for the purpose of illustrating a portion of the medical instrument that forms no part of the claimed design.