The disease characterized by extreme muscular weakness and brownish pigmentation of the buccal cavity and skin is

The correct answer is Addison’s disease.

Addison’s disease (also known as primary adrenal insufficiency) is an endocrine disorder that occurs when the adrenal glands do not produce sufficient amounts of steroid hormones, primarily cortisol and aldosterone.

The characteristic symptoms mentioned in the question can be explained as follows:

1. Brownish pigmentation of the buccal cavity and skin (Hyperpigmentation): Due to the lack of cortisol, the pituitary gland compensates by overproducing adrenocorticotropic hormone (ACTH). The precursor molecule for ACTH is pro-opiomelanocortin (POMC). When POMC is cleaved to yield ACTH, it also produces melanocyte-stimulating hormone (MSH). Elevated levels of MSH and ACTH stimulate melanocytes in the skin and mucous membranes, leading to characteristic bronze-like or brownish pigmentation, particularly in areas exposed to friction, pressure, and in the buccal cavity (inside the mouth).

2. Extreme muscular weakness and fatigue: Cortisol plays a vital role in maintaining blood pressure, vascular reactivity, and regulating carbohydrate, protein, and lipid metabolism. Aldosterone helps maintain sodium and water balance. Deficiency in these hormones leads to muscle wasting, chronic fatigue, muscle weakness, and low blood pressure (hypotension).

Let's briefly review why the other options are incorrect:

• Grave’s disease: This is an autoimmune disorder characterized by hyperthyroidism (overactive thyroid), leading to symptoms like weight loss, rapid heart rate, heat intolerance, and bulging eyes (exophthalmos), rather than hyperpigmentation.

• Myxoedema: This is a condition associated with severe hypothyroidism (underactive thyroid) in adults, characterized by a dry, waxy swelling of the skin, cognitive slowing, and lethargy.

• Cushing’s disease: This is caused by an excess of cortisol (hypercortisolism), leading to symptoms like weight gain, a "buffalo hump," a round "moon face," and thinning skin with purple striae, which is the opposite of the hormone deficiency seen in Addison's disease.