**Core Concept**
A simple lentigo is a type of benign melanocytic lesion characterized by an increase in the number of melanocytes, the pigment-producing cells in the epidermis. This proliferation of melanocytes leads to an accumulation of melanin, resulting in a well-circumscribed, darkly pigmented macule.

**Why the Correct Answer is Right**
The correct answer, **B. Increased melanocytes**, is the primary reason for the development of a simple lentigo. This increase in melanocytes is typically seen in response to UV radiation, which stimulates the proliferation of melanocytes. The resulting lesion consists of an aggregation of melanocytes in the epidermis, often in a linear or radial pattern. This increase in melanocytes leads to an accumulation of melanin, which gives the lesion its characteristic dark color.

**Why Each Wrong Option is Incorrect**
* **Option A:** Increased melanin is a consequence of the increased melanocytes, not the primary cause of a simple lentigo. While melanin accumulation is a key feature of the lesion, it is not the underlying reason for its development.
* **Option C:** Increased melanosomes are also a consequence of the increased melanocytes. Melanosomes are the organelles within melanocytes that contain and package melanin. While the number of melanosomes may increase in response to the increased melanocytes, this is not the primary cause of the lesion's development.
* **Option D:** All of the above is incorrect because the primary cause of a simple lentigo is the increase in melanocytes, not the increase in melanin or melanosomes.

**Clinical Pearl / High-Yield Fact**
It is essential to differentiate simple lentigines from other types of melanocytic lesions, such as compound or dysplastic nevi, which may have a more complex histopathology and a higher risk of malignant transformation.

✓ Correct Answer: B. Increased melanocytes.