What is the first step you should take when assisting someone who has fallen from a ladder and complains of back pain?

When assisting someone who has fallen from a ladder and is complaining of back pain, the priority is to prevent any further injury. Having the person remain as still as possible is essential in this situation. Movement can exacerbate potential spinal injuries, which could lead to serious complications, including paralysis.

Keeping the individual still minimizes the risk of additional damage to the spine and surrounding structures. It's vital to ensure that they do not attempt to move or get up until they have been properly assessed by a medical professional. This approach aligns with standard first aid protocols for suspected spinal injuries, allowing for a safer response to the situation.

In the context of the other options, encouraging movement or attempting to check for mobility can lead to worsened injuries. Immediate assessment of visible injuries may distract from the critical need to stabilize the person. Thus, maintaining stillness is the best course of action until professional help arrives.