Deep Vein Thrombosis (DVT): This is the most likely diagnosis due to the combination of unilateral lower extremity edema and posterior calf pain, which are classic symptoms of DVT. The pain often worsens with walking or standing.

Muscle Strain: Overuse or direct injury to the muscles in the calf can cause pain and swelling, mimicking the symptoms of DVT.
Cellulitis: A bacterial infection of the skin and subcutaneous tissues that can cause redness, warmth, swelling, and pain in the affected limb.
Lymphedema: Although typically bilateral, lymphedema can present unilaterally and cause significant swelling of the affected limb.

Popliteal Artery Aneurysm or Thrombosis: These conditions can cause severe leg pain and swelling due to compromised arterial blood flow. Missing these diagnoses can lead to severe consequences, including limb loss.
Compartment Syndrome: A condition where increased pressure within a muscle compartment leads to inadequate blood supply to the tissues within that space. It is a medical emergency that requires prompt intervention.
Phlegmasia Cerulea Dolens: A severe form of DVT that causes significant swelling and cyanosis of the limb, which can rapidly progress to gangrene if not treated promptly.

Lipedema: A chronic condition characterized by the abnormal growth of fat cells in the legs, leading to swelling and pain. It is often mistaken for lymphedema but does not typically cause posterior calf pain.
Chronic Venous Insufficiency: While not rare, it is less likely to present acutely with significant posterior calf pain. It causes swelling, skin changes, and pain due to the veins' inability to send blood back to the heart from the legs.
Neoplastic or Paraneoplastic Conditions: Certain cancers can cause venous or lymphatic obstruction leading to unilateral limb swelling and pain. These are rare but important considerations in the differential diagnosis.