Deep Vein Thrombosis (DVT): This is a common cause of unilateral leg swelling, especially in the absence of injury. The patient's age and the acute onset of symptoms without trauma make DVT a leading consideration.

Cellulitis: An infection of the skin and subcutaneous tissues that can cause swelling, redness, and warmth of the affected area. It's a common condition that could present with thigh swelling.
Lymphedema: Although less common, lymphedema could cause unilateral swelling, especially if there's a history of surgery, radiation, or infection affecting lymph nodes.
Muscle Strain or Contusion: Even without a reported injury, minor trauma or overuse could lead to muscle strain or contusion, causing swelling.

Malignancy (e.g., Soft Tissue Sarcoma): Although rare, any unexplained swelling, especially if persistent or worsening, warrants consideration of malignancy to avoid delayed diagnosis.
Compartment Syndrome: A medical emergency that could present with swelling, pain, and decreased mobility. It's crucial to identify and treat promptly to avoid long-term damage.
Popliteal Artery Aneurysm or Arteriovenous Malformation: These vascular abnormalities can cause swelling and require urgent attention to prevent serious complications.

Lipedema: A chronic condition characterized by the abnormal growth of fat cells in the legs, leading to swelling. It's less common and often misdiagnosed as lymphedema.
Filarial Infection: In endemic areas, filarial infections can cause lymphatic obstruction leading to swelling. This would be an unusual diagnosis in most Western countries without travel history.
Pseudolymphoma or Other Rare Inflammatory Conditions: Conditions like pseudolymphoma or other rare inflammatory diseases could present with swelling but are much less common and typically have additional symptoms.