**Core Concept:** Hyperthyroidism is a condition characterized by excess thyroid hormones produced by the thyroid gland, leading to increased basal and physical activity of the thyroid cells. This excess production results in goiter formation and calcium deposits in the thyroid capsule.

**Why the Correct Answer is Right:** The correct answer, D, refers to thyroiditis, which is an inflammation of the thyroid gland. In this case, it is Graves' thyroiditis, a type of autoimmune thyroid disease. The autoantibodies in Graves' disease cause the thyroid to produce excessive thyroid hormones, leading to the presentation described (increased basal and physical activity, goiter, and calcium deposits in the thyroid capsule).

**Why Each Wrong Option is Incorrect:**

A. Myxedema: This refers to hypothyroidism, which is characterized by reduced thyroid hormone production. It does not explain the increased basal and physical activity, goiter, and calcium deposits in the thyroid capsule.

B. Subacute thyroiditis: This is a self-limiting inflammation of the thyroid gland, usually caused by a viral infection or autoimmune response. While it does cause hyperthyroidism due to thyroid hormone release, it does not explain the presence of goiter and calcium deposits in the thyroid capsule.

C. Hashimoto's thyroiditis: This is another autoimmune thyroid disease causing hypothyroidism, not hyperthyroidism. It does not explain the increased basal and physical activity, goiter, and calcium deposits in the thyroid capsule.

**Clinical Pearl:** Hyperthyroidism due to Graves' thyroiditis is a common cause of increased basal and physical activity, goiter, and calcium deposits in the thyroid capsule. The presence of these signs and symptoms should prompt a thorough workup for hyperthyroidism, including autoimmune markers.

**Correct Answer:** D. Graves' thyroiditis