Hematoma: This is the most likely diagnosis given the history of a fall onto a stair edge, which can cause significant blunt trauma to the leg. A hematoma, a collection of blood outside of blood vessels, can form as a result of this trauma, presenting as a lump.

Bruise (Contusion): Similar to a hematoma, a bruise can occur from the same mechanism of injury. It might not present as a distinct lump but could be part of the differential for a lump if it's particularly large or if there's significant swelling.
Soft Tissue Injury: This includes injuries to muscles, tendons, and ligaments. A fall could cause strains or tears in these tissues, leading to swelling and potentially a palpable lump.
Lipoma or Other Pre-existing Benign Tumors: If the lump was present before the fall but became more noticeable due to swelling or attention drawn to the area, a pre-existing benign tumor like a lipoma could be considered.

Compartment Syndrome: Although less likely, compartment syndrome is a serious condition that can occur after trauma to a limb. It involves increased pressure within a muscle compartment, which can lead to muscle and nerve damage. Early diagnosis is crucial.
Deep Vein Thrombosis (DVT): While more commonly associated with immobility, a DVT could potentially occur after a significant injury, especially if there's prolonged immobilization or significant trauma to the veins.
Osteosarcoma or Other Malignant Tumors: Although very rare and unlikely to be directly caused by a fall, if a lump is present and not clearly explained by trauma, malignant tumors must be considered, especially in certain age groups or with specific risk factors.

Pseudolipoma (Morel-Lavallée Lesion): A rare condition where a lump forms due to the separation of skin and fascia from the underlying tissue, often as a result of severe trauma.
Foreign Body Reaction: If the fall involved contact with a foreign object that penetrated the skin, a lump could form as a reaction to the foreign body.
Infection or Abscess: While less common as an immediate result of a fall, if the skin was broken during the fall, there's a risk of infection, which could present as a lump.