Muscle mostly affected in thyroid ophthalmopathy is:
**Core Concept**
Thyroid ophthalmopathy, also known as Graves' orbitopathy, is a condition characterized by inflammation and swelling of the tissues around the eye, including the muscles, eyelids, and lacrimal gland. This condition is often associated with Graves' disease, an autoimmune thyroid disorder. The extraocular muscles are primarily affected, leading to eyelid retraction, exophthalmos, and diplopia.
**Why the Correct Answer is Right**
The extraocular muscles are the primary site of inflammation in thyroid ophthalmopathy. The inferior rectus muscle is the most commonly affected, followed by the medial rectus, superior rectus, and lateral rectus muscles. The inflammation leads to fibrosis and scarring of the muscles, resulting in restricted eye movements and diplopia. The involvement of these muscles is due to the autoimmune response against the thyroid-stimulating hormone receptor (TSH-R), which is also expressed on the surface of the orbital tissues.
**Why Each Wrong Option is Incorrect**
* **Option A:** The extraocular muscles are not the primary site of inflammation in thyroid ophthalmopathy, although they may be affected.
* **Option B:** The lacrimal gland is involved in thyroid ophthalmopathy, but it is not the muscle most affected.
* **Option D:** The orbital fat is swollen in thyroid ophthalmopathy, but it is not the muscle most affected.
**Clinical Pearl / High-Yield Fact**
The inferior rectus muscle is the most commonly affected muscle in thyroid ophthalmopathy, and its involvement can lead to significant morbidity, including diplopia and vision loss. Early recognition and management of thyroid ophthalmopathy are crucial to prevent long-term complications.
**Correct Answer:** C.