What is a primary use for a hard knife in medicine?

A hard knife, often referred to in medical contexts as a surgical scalpel or cutting instrument, is primarily used to treat physical injuries by making incisions or excisions on the body. It enables healthcare providers to access tissues during surgery or to dissect areas that may be affected by trauma or disease. This ability to create precise cuts allows for better treatment of wounds as well as performance of surgeries on various body parts, which is critical in fixing physical injuries like bumps or other conditions that require surgical intervention.

In contrast, the other options do not involve the use of a hard knife in a medical sense. Treating constipation typically involves medications or dietary changes rather than surgical instruments. Cleaning scratches commonly uses antiseptics or bandages, and polishing diuretics does not relate to the function of a knife at all, as diuretics are medications and not treated with surgical instruments. Thus, the primary use of a hard knife in the medical field aligns with its role in addressing physical injuries.