Intertrigo: This is the most common diagnosis for shiny red skin at skin folds, especially in infants. It is caused by friction, heat, and moisture, which creates an ideal environment for bacterial and fungal growth. The condition worsens with sweating and heat, fitting the description provided.

Eczema (Atopic Dermatitis): While more commonly associated with dry, itchy skin, eczema can also present with red, shiny skin in skin folds, particularly in infants. It can be exacerbated by heat and sweating.
Seborrhoeic Dermatitis: This condition can cause red, oily, scaly patches on the skin, including in skin folds. It's common in infants and can be irritated by heat and moisture.
Candidal Infection: Candida thrives in warm, moist environments, making skin folds a common site for infection, especially in infants who wear diapers. The infection can cause shiny red skin that worsens with sweating and heat.

Staphylococcal Scalded Skin Syndrome (SSSS): Although less common, SSSS is a serious condition caused by staphylococcal toxins that can lead to widespread skin lesions and can be life-threatening. It typically starts with red, tender skin that can resemble a burn.
Toxic Epidermal Necrolysis (TEN): A severe skin condition usually triggered by medications, TEN can cause widespread skin necrosis and detachment. It's rare in infants but can be fatal if not recognized and treated promptly.

Langerhans Cell Histiocytosis: A rare disorder in which the body accumulates too many immature Langerhans cells, leading to various symptoms, including skin lesions. While rare, it can present with skin manifestations that might resemble other conditions.
Zinc Deficiency: Can cause skin lesions and may present with red, shiny skin, particularly in areas of skin folds. It's less common but should be considered, especially if the infant has a diet deficient in zinc or has malabsorption issues.