Trauma or Contusion: The most likely cause of bruising near the sacrum with sciatica is a traumatic injury, such as a fall, which can cause both bruising and irritation of the sciatic nerve, leading to sciatica.

Hematoma: A collection of blood outside of blood vessels, which can occur due to trauma or bleeding disorders, and can cause pressure on the sciatic nerve, leading to sciatica.
Deep Vein Thrombosis (DVT): Although less common in the pelvic area, DVT can cause swelling, bruising, and pain, which might radiate along the path of the sciatic nerve.
Musculoskeletal Injury: Strains or tears in the muscles or ligaments around the sacrum can cause bruising and sciatica due to inflammation and irritation of the nerve.

Cauda Equina Syndrome: A serious condition that occurs when the nerves in the spinal canal are compressed, which can be caused by a herniated disk, trauma, or tumor. It requires immediate medical attention to prevent permanent damage.
Spinal Epidural Hematoma: A rare but potentially life-threatening condition where bleeding occurs in the epidural space surrounding the spinal cord, which can compress the spinal cord and nerves, including the sciatic nerve.
Osteomyelitis or Discitis: Infections of the bone or intervertebral disc, respectively, which can cause pain, bruising, and neurological symptoms due to the proximity to the sciatic nerve.

Sacral Insufficiency Fracture: A type of stress fracture that can occur in the sacrum, often in individuals with osteoporosis or other bone diseases, which can cause pain and bruising.
Neurogenic Tumors: Tumors that arise from the nerve tissues, such as neurofibromas or schwannomas, which can compress the sciatic nerve and cause symptoms.
Vascular Malformations: Abnormal formations of blood vessels that can cause bleeding, bruising, and neurological symptoms if they compress or bleed onto nearby nerves.