During an iteration a planned activity becomes more complex but must be delivered as quickly as possible. Which approach should be used?

When an activity becomes more complex but still needs to be delivered quickly within an iteration, the most effective approach in an agile environment is for the cross-functional team members to collaborate and leverage their diverse skills to complete the activity. This embodies the agile principle of self-organizing teams. Obtaining support from external team members or bringing in a specialist might introduce delays or dependencies, contradicting the need for quick delivery. Moving the work item to the product backlog would defer the work, which is not suitable when it "must be delivered as quickly as possible" within the current iteration.