Final answer:
The inferior rectus muscle is the most affected extraocular muscle in thyroid ophthalmopathy, a condition often associated with hyperthyroidism and Graves' disease. This condition can lead to proptosis, lid retraction, and restrictive strabismus, where the range of eye movement is limited.
Step-by-step explanation:
The most affected individual extraocular muscle in thyroid ophthalmopathy is the inferior rectus muscle. Thyroid ophthalmopathy, also known as Graves' ophthalmopathy or exophthalmia, is a condition associated with thyroid dysfunction, particularly hyperthyroidism and Graves' disease. In this condition, inflammation and swelling affect the muscles and tissues around the eyes, but the inferior rectus is often the most significantly involved. This can lead to symptoms such as proptosis (bulging of the eyes), lid retraction, discomfort, and double vision.
The extraocular muscles are a group of six muscles that control eye movement. The inferior rectus, like most of the extraocular muscles, is innervated by the oculomotor nerve. Involvement of these muscles in thyroid ophthalmopathy can lead to restrictive strabismus, where the range of eye movement is limited due to the fibrosis of the muscles, often most prominently affecting the inferior rectus.