Moisture-associated skin damage (MASD) is a broad term encompassing skin injury caused by prolonged exposure to various sources of moisture, including urine, stool, perspiration, wound exudate, stomal effluent, and saliva. MASD represents a significant and preventable source of patient discomfort, increased infection risk, and healthcare costs. Understanding the pathophysiology of MASD requires knowledge of normal skin structure and function. The skin is the body's largest organ and serves as the primary barrier against the external environment. The stratum corneum (outermost layer of the epidermis) consists of flattened, keratinized cells (corneocytes) embedded in a lipid matrix, often described as a brick-and-mortar arrangement. This structure provides the critical barrier function that prevents water loss, protects against chemical penetration, and inhibits microbial invasion. The skin maintains a normal acidic pH of 4.0 to 6.5, often called the acid mantle, which inhibits bacterial and fungal colonization and supports the activity of enzymes essential for lipid barrier maintenance. When skin is exposed to excessive moisture, several pathological processes occur simultaneously. First, overhydration (maceration) causes the stratum corneum to absorb water and swell, weakening intercellular connections...