Musculoskeletal Injury: This is often the most common cause of flank pain and ecchymosis, resulting from direct trauma to the muscles or bones in the flank area, leading to bruising and pain.

Kidney Stone (Nephrolithiasis): While kidney stones can cause severe pain, the presence of ecchymosis might be less common unless there's an associated injury or severe inflammation. However, it's a plausible cause for flank pain.
Pyelonephritis: An infection of the kidney, which can cause flank pain and potentially ecchymosis if there's an associated abscess or severe inflammation.
Hematoma: A collection of blood outside of blood cells, which could be due to trauma or a spontaneous bleed, causing both pain and ecchymosis in the flank area.

Retroperitoneal Hemorrhage: A potentially life-threatening condition where there is bleeding into the space behind the peritoneum, which could be due to trauma, anticoagulation, or vascular diseases. It requires immediate medical attention.
Abdominal Aortic Aneurysm (AAA): Although AAA typically presents with abdominal pain, a leaking or ruptured aneurysm could cause flank pain and ecchymosis, especially if the hematoma tracks down to the flank area.
Pulmonary Embolism: While less directly related to flank pain and ecchymosis, in rare cases, a pulmonary embolism could cause referred pain to the flank area, especially if there's associated hypoxia or other systemic symptoms.

Wilm's Tumor: A rare kidney cancer that could cause a mass effect leading to pain and potentially ecchymosis if the tumor invades or compresses surrounding structures.
Lymphoma: Certain types of lymphoma could involve the kidneys or retroperitoneal lymph nodes, leading to flank pain and potentially ecchymosis if there's significant tumor burden or bleeding.
Vasculitis: Inflammatory diseases affecting blood vessels, such as polyarteritis nodosa, could potentially cause flank pain and ecchymosis due to vascular inflammation and damage.