**Core Concept:** Penetration of drugs through biological membranes, particularly intact skin, is essential for understanding their pharmacokinetics and therapeutic effects.

**Why the Correct Answer is Right:** Penetration of a drug through the skin depends on its physicochemical properties, such as molecular size, lipophilicity, and polarity. Drugs must have a sufficient lipophilic nature to cross the skin's lipid bilayer, which is made up of sebum and ceramides.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because it fails to consider the drug's lipophilicity. Hydrocortisone is a steroid with a high degree of lipophilicity, which allows it to penetrate through intact skin.

B. Option B is incorrect because insulin is a peptide hormone, and peptides usually have poor lipophilicity. Peptides generally require formulation with adjuvants like oils, solvents, or permeation enhancers to increase skin penetration.

C. This option is wrong due to the same reason as option A. Penetration enhancers, which are added to improve skin permeation, would be required for drugs with poor lipophilicity, like aspirin. However, aspirin is a lipophilic drug and can penetrate intact skin without enhancers.

D. Liposomes are vesicles that can enhance drug penetration into the skin by increasing the drug's lipophilicity. Although liposomes can improve the penetration of some drugs, they are not inherently incapable of penetrating intact skin.

**Clinical Pearl:** Understanding drug penetration through skin is crucial in formulating topical medications. Penetration enhancers, like liposomes or permeation enhancers, can significantly improve the efficacy of topical drugs, but they are not always necessary.

**Correct Answer:** D. Liposomes are vesicles that can enhance drug penetration into the skin by increasing the drug's lipophilicity. Although liposomes can improve the penetration of some drugs, they are not inherently incapable of penetrating intact skin.