**Core Concept**
Hyperkeratinization is a process where there is an abnormal thickening of the stratum corneum, the outermost layer of the epidermis, due to an overproduction of keratin. This condition can lead to various skin disorders, including ichthyosis and psoriasis.

**Why the Correct Answer is Right**
Hyperkeratinization is often caused by mutations in genes that regulate keratinization, such as the filaggrin gene. The filaggrin protein plays a crucial role in the normal desquamation process of the skin, and its deficiency can lead to hyperkeratinization. Additionally, hyperkeratinization can be triggered by external factors such as dry skin, irritation, or cold temperatures.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because hyperkeratinization is not primarily associated with increased cell turnover, which is more characteristic of conditions like psoriasis.
**Option B:** This option is incorrect because hyperkeratinization is not primarily associated with a decrease in keratin production, which would be more characteristic of conditions like ichthyosis.
**Option C:** This option is incorrect because hyperkeratinization is not primarily associated with increased melanin production, which would be more characteristic of conditions like melasma.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that hyperkeratinization can be a feature of several skin disorders, including ichthyosis, psoriasis, and keratosis pilaris. It is essential to consider the clinical presentation and history when diagnosing these conditions.

**Correct Answer: D. Hyperkeratinization is a hallmark feature of psoriasis.**