Final answer:
The correct word to fill in the blank is 'intensity,' as it describes how the strength of the pain Mark felt increased over time.
Step-by-step explanation:
The wound didn't hurt at first, but the intensity of the pain increased as time went on. After a skin injury, our bodies start a wound-healing response. Initially, there might not be much pain, but as the body sends more blood to the area to aid healing, substances like histamine increase blood flow, leading to swelling, redness, and warmth, which can make the area hurt more. Mark's situation was more severe; he developed a gangrenous infection, making the pain, redness, and swelling much worse, and requiring urgent medical attention.