**Core Concept**
Back washing is a sterile technique used to remove any debris or contaminants from the hub of an intravenous (IV) cannula or needle after its initial use. This is crucial to prevent the introduction of bacteria into the vascular system, thereby reducing the risk of infection and phlebitis.

**Why the Correct Answer is Right**
The correct answer is related to the importance of maintaining asepsis during IV cannulation. When a cannula or needle is inserted into a vein, it can introduce bacteria into the bloodstream, leading to phlebitis or sepsis. Back washing involves flushing the hub of the cannula or needle with a sterile solution to remove any debris or contaminants that may have accumulated during the insertion process. This helps to maintain the sterility of the IV access site and reduces the risk of infection.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because back washing is not typically used for cleaning the skin before IV cannulation. Instead, the skin is usually cleaned with an antiseptic solution to reduce the risk of infection.

**Option B:** This option is incorrect because back washing is not a technique used for removing air from the IV tubing. Air embolism can occur when air enters the IV tubing, but back washing is not an effective method for removing air.

**Option C:** This option is incorrect because back washing is not a technique used for flushing the IV catheter itself. While flushing the IV catheter is an important part of maintaining IV access, back washing is specifically used to clean the hub of the cannula or needle.

**Clinical Pearl / High-Yield Fact**
To remember the importance of back washing, recall the phrase "Clean the hub, not the rest!" This emphasizes the specific goal of back washing, which is to maintain the sterility of the hub of the cannula or needle.

**Correct Answer:** D.