Balanitis: This is an inflammation of the glans penis and is the most common condition affecting the foreskin. It can be caused by poor hygiene, infections (bacterial, fungal, or viral), or allergic reactions. The symptoms include redness, swelling, and sometimes discharge, making it the most likely diagnosis for redness and swelling of the foreskin.

Phimosis: A condition where the foreskin cannot be fully retracted over the glans penis. It can lead to swelling and redness due to the inability to properly clean the area, leading to infection or irritation.
Paraphimosis: This occurs when the foreskin is retracted behind the glans and cannot return to its original position, causing swelling and potentially cutting off blood flow. It's a painful condition that requires immediate attention.
Contact Dermatitis: An allergic reaction or irritation from soaps, condoms, or other products can cause redness and swelling of the foreskin.
Urinary Tract Infection (UTI): While less common in men, a UTI can cause irritation and inflammation of the genital area, including the foreskin.

Gangrene or Necrotizing Fasciitis: Although rare, these conditions can arise from untreated paraphimosis or severe infections. They are medical emergencies that can lead to severe damage or death if not promptly treated.
Cancer: While extremely rare, penile cancer can present with changes in the skin of the penis, including the foreskin, such as redness, swelling, or ulcers.

Behçet's Disease: A rare condition that can cause ulcers and inflammation in various parts of the body, including the genital area.
Fixed Drug Eruption: A rare skin condition caused by an adverse reaction to medication, which can cause well-defined, red, swollen patches on the skin, including the foreskin.
Lichen Sclerosus: A rare skin condition that creates patchy, white skin that's thinner than normal. It can cause itching, pain, and swelling, especially if it affects the foreskin.