The most impoant factor governing absorption of a drug from intact skin is
**Core Concept**
The absorption of a drug from intact skin is influenced by several factors, but the most critical one is the lipophilicity of the drug. This is due to the nature of the skin's barrier function, which is primarily composed of lipids and cellular membranes.
**Why the Correct Answer is Right**
The lipophilicity of a drug determines its ability to cross the skin's barrier, which is composed of stratum corneum, a layer of lipids and cellular membranes. The stratum corneum acts as a diffusion barrier, and the lipophilicity of the drug affects its ability to dissolve in this lipid-rich layer. Drugs with high lipophilicity can easily dissolve in the lipids and traverse the skin, whereas hydrophilic drugs have difficulty penetrating the skin's barrier. This is because the skin's natural moisturizing factor and the hydrophilic nature of the stratum corneum's cellular membranes hinder the passage of hydrophilic compounds.
**Why Each Wrong Option is Incorrect**
**Option A:** While the molecular weight of a drug can impact its absorption, it is not the most critical factor governing absorption from intact skin.
**Option B:** The surface area of application can influence the absorption of a drug, but it is not the most important factor.
**Option C:** The concentration of a drug can affect its absorption, but it is not the primary factor governing absorption from intact skin.
**Clinical Pearl / High-Yield Fact**
When developing topical formulations, pharmaceutical companies often focus on enhancing the lipophilicity of the drug to improve its absorption through the skin. This can be achieved through various methods, such as using lipid-based excipients or modifying the drug's molecular structure.
**Correct Answer: C. Lipophilicity**