A first medical instrument includes a flexible catheter and a medical guidewire extendable beyond a distal end of the catheter. The medical guidewire has first and second segments, wherein the bending moment of inertia of the first segment is less than the bending moment of inertia of the second segment. A second medical instrument includes a flexible catheter, a mechanized guidewire assembly, and a medical guidewire extendable beyond a distal end of the catheter. The medical guidewire has an exterior surface including a repetitive series of spaced-apart surface elevation features. One example of surface elevation features is external threads. The spaced-apart surface elevation features are adapted for operable engagement with the mechanized guidewire drive assembly.