Ankle Sprain: The patient's history of rolling her ankle and subsequent swelling is highly suggestive of an ankle sprain, which is a common injury that occurs when the ligaments surrounding the ankle joint are stretched or torn.

Ankle Contusion: Although the patient reports no bruising, it's possible that the swelling could be due to a contusion (bruise) of the soft tissues around the ankle.
Deltoid Ligament Sprain: The deltoid ligament is located on the medial (inner) aspect of the ankle and can be injured when the ankle is rolled, leading to swelling and pain.
Peroneal Tendon Strain: The peroneal tendons run along the lateral (outer) aspect of the ankle and can be strained or torn when the ankle is rolled, leading to swelling and pain.

Fracture (e.g. Maisonneuve Fracture): Although less likely, it's essential to consider the possibility of a fracture, particularly a Maisonneuve fracture, which is a fracture of the proximal fibula that can occur when the ankle is rolled.
Osteochondral Lesion: An osteochondral lesion is a type of injury that affects the cartilage and bone of the ankle joint and can occur when the ankle is rolled.
Compartment Syndrome: Compartment syndrome is a rare but potentially serious condition that can occur when there is increased pressure within the compartments of the leg, leading to pain, swelling, and numbness.

Gout or Pseudogout: Although rare in an 18-year-old, it's possible that the patient could have a crystal arthropathy such as gout or pseudogout, which can cause sudden onset of swelling and pain in the ankle.
Infectious Arthritis: Infectious arthritis is a rare but potentially serious condition that can cause swelling, pain, and warmth in the affected joint.
Tumor or Cyst: A tumor or cyst in the ankle joint or surrounding tissues is a rare possibility that could cause swelling and pain.