## Core Concept
The question tests knowledge of conditions associated with pigmentation changes in both the oral cavity and the skin. These conditions often involve systemic diseases or syndromes that manifest with pigmentation due to various pathophysiological mechanisms.

## Why the Correct Answer is Right
Addison's disease, caused by adrenal insufficiency, leads to increased ACTH levels, which stimulates melanocortin receptors in melanocytes, causing pigmentation in the skin and oral mucosa. Peutz-Jeghers syndrome is characterized by hamartomatous polyps in the gastrointestinal tract and mucocutaneous melanin deposits. Neurofibromatosis type 1 (NF1) can cause café-au-lait spots (skin) and oral neurofibromas or pigmentation. However, **C. Cushing's syndrome** primarily presents with weight gain, hypertension, and glucose intolerance due to excess cortisol, not typically with pigmentation in both skin and oral cavity as a hallmark feature.

## Why Each Wrong Option is Incorrect
* **Option A: Addison's disease** is incorrect because it indeed causes pigmentation in both the skin and oral cavity due to increased melanocyte-stimulating hormone (MSH) activity.
* **Option B: Peutz-Jeghers syndrome** is incorrect as it characteristically presents with melanin deposits in the skin and oral mucosa.
* **Option D: Neurofibromatosis** is incorrect because it can cause pigmentation changes, including café-au-lait spots in the skin and sometimes oral manifestations.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that Addison's disease can cause a diffuse darkening of the skin and mucous membranes, including the oral cavity, which is a critical diagnostic clue. Conversely, Cushing's syndrome, while causing many systemic effects, does not characteristically cause such pigmentation changes.

## Correct Answer: C. Cushing's syndrome