What first-aid step should you take if you accidentally brush against a jellyfish?

When dealing with a jellyfish sting, it is crucial to neutralize the nematocysts (stinging cells) that may still be active on the skin. Rinsing the area with acetic acid or vinegar is the recommended first-aid step because it helps to inactivate the stinging cells, preventing further release of venom and alleviating pain. Vinegar's acidity is effective in neutralizing certain types of jellyfish stings, particularly those from species like the box jellyfish.

In contrast, using fresh water can trigger the nematocysts to release more venom, which could worsen the sting. Rubbing the area vigorously risks breaking the stinging cells, which again can cause more venom to enter the body. Applying heat packs can provide relief for some types of stings but is not a first-aid priority when dealing with jellyfish; it is typically suggested after appropriate rinsing to ease pain.

Therefore, rinsing the area with acetic acid or vinegar is the correct approach as it directly addresses the immediate concern of neutralizing the venom and protecting the injured area from further harm.