Cellulitis: This is a common condition characterized by bacterial infection of the skin and subcutaneous tissues, often presenting with swelling, redness, warmth, and pain in the affected area. The thighs are a common site for cellulitis due to trauma, insect bites, or other breaks in the skin.

Deep Vein Thrombosis (DVT): Swelling in one thigh could be a sign of DVT, which is a blood clot in a deep vein. Risk factors include recent surgery, immobility, cancer, and genetic predispositions.
Lymphedema: This condition involves swelling due to impaired lymph drainage. It can be primary (inherited) or secondary (due to surgery, infection, or trauma).
Muscle Strain or Contusion: Direct injury to the thigh muscles can cause swelling, pain, and bruising.
Insect Bite or Sting Reaction: Some individuals may have a significant allergic reaction to insect bites or stings, leading to localized swelling.

Compartment Syndrome: A medical emergency where increased pressure within a muscle compartment leads to inadequate blood supply to the tissues. It presents with severe pain, swelling, and sometimes pallor or paresthesia of the affected limb.
Necrotizing Fasciitis: A severe infection of the fascia, a layer of tissue that covers muscles, nerves, fat, and blood vessels in the affected area. It is characterized by severe pain, swelling, and sometimes skin necrosis.
Malignancy (e.g., Soft Tissue Sarcoma): Although rare, swelling in the thigh could be a presenting symptom of a soft tissue tumor.

Filariasis: In areas where it is endemic, filariasis can cause lymphedema and swelling, typically in the legs.
Pseudogout (Calcium Pyrophosphate Deposition Disease): Can cause acute inflammatory arthritis and swelling, though it more commonly affects the knees.
Fabry Disease: A genetic disorder that can lead to swelling in the extremities due to the accumulation of a particular type of fat in the body's cells.
Lipedema: A chronic condition characterized by the abnormal growth of fat cells in the legs, leading to swelling and pain.