Trauma: The most straightforward explanation for unexplained bruising and swelling of the foot is some form of trauma, which could be acute or repetitive. This could range from a direct injury to overuse in sports or activities that involve running or repetitive foot striking.

Vascular Insufficiency: Conditions such as peripheral arterial disease or venous insufficiency can lead to bruising and swelling due to poor circulation.
Infection: Cellulitis, an infection of the skin and subcutaneous tissues, can cause swelling, redness, and sometimes bruising.
Gout or Pseudogout: These are forms of arthritis that can cause sudden, severe swelling and may be accompanied by bruising, especially if there has been trauma to the joint.
Medication Side Effects: Certain medications, such as anticoagulants, can increase the risk of bruising.

Deep Vein Thrombosis (DVT): Although less common, DVT is a potentially life-threatening condition that can cause swelling and bruising of the foot and leg. It requires immediate medical attention.
Osteomyelitis: A bone infection that can cause swelling, pain, and sometimes bruising. It's a serious condition that needs prompt treatment.
Compartment Syndrome: A condition resulting from increased pressure within a muscle compartment, which can cause pain, swelling, and bruising. It's a medical emergency.

Hematologic Disorders: Conditions such as hemophilia or other bleeding disorders can lead to easy bruising and swelling.
Amyloidosis: A group of diseases that can cause the accumulation of abnormal proteins (amyloid) in various tissues, potentially leading to bruising and swelling.
Ehlers-Danlos Syndrome: A genetic disorder that affects the body's connective tissue and can lead to easy bruising, among other symptoms.
Scurvy: A disease resulting from vitamin C deficiency, which can cause bruising and swelling among other symptoms. It's rare in developed countries but can occur in individuals with poor nutrition.