Cellulitis: This is a common infection of the skin and subcutaneous tissues, often caused by streptococcal or staphylococcal bacteria. The presentation of erythematous, swollen, red, and tender skin, particularly in an elderly individual, is highly suggestive of cellulitis. The fact that it involves the lower extremities up to the mid-shin also aligns with the typical distribution of cellulitis.

Deep Vein Thrombosis (DVT): While DVT typically presents with unilateral leg swelling, pain, and discoloration, it could be considered in the differential, especially if there's a history of recent immobility, surgery, or cancer. However, bilateral involvement and the specific pattern of erythema and tenderness might make this less likely.
Eczema/Dermatitis: Certain types of dermatitis, such as stasis dermatitis or contact dermatitis, could present with erythema and swelling, especially in the context of poor circulation or allergic reactions. However, the acute onset and specific distribution might not fully align with these conditions.
Gout or Pseudogout: These conditions can cause acute, painful swelling of the joints, including the knees and potentially the feet. However, they typically present with more localized joint involvement and might not fully explain the diffuse erythema and swelling up to the mid-shin.

Necrotizing Fasciitis: Although rare, this is a life-threatening condition that requires immediate recognition and treatment. It can present with severe pain, swelling, and erythema of the affected limb, often with a history of trauma or recent surgery. The elderly are at higher risk due to potential comorbidities and decreased mobility.
Septic Arthritis: If the knee is significantly involved, septic arthritis should be considered, especially if there's a history of joint disease or recent joint injection. This condition requires prompt diagnosis and treatment to prevent joint destruction.
Compartment Syndrome: This is another emergency condition characterized by increased pressure within a muscle compartment, leading to pain, swelling, and potentially neurovascular compromise. It might not initially present with the described erythema but could evolve to include these symptoms.

Erythema Nodosum: This condition presents with tender, erythematous nodules typically on the lower extremities. While it could be considered, the description provided does not specifically mention nodules, and the distribution might not fully align.
Complex Regional Pain Syndrome (CRPS): Formerly known as reflex sympathetic dystrophy, CRPS can cause swelling, pain, and changes in skin temperature and color. However, it usually follows an injury and has a more complex set of symptoms, including significant pain out of proportion to the injury and autonomic dysfunction.