**Core Concept**
Thyroid ophthalmopathy is a condition associated with Graves' disease, characterized by inflammation and swelling of the tissues around the eye, leading to various ocular symptoms. The earliest symptoms often precede the development of more noticeable signs like exophthalmos.

**Why the Correct Answer is Right**
The correct answer is related to the initial presentation of thyroid ophthalmopathy, which typically begins with mild non-specific symptoms. These symptoms include mild lid lag, sensitivity to light, and mild eye discomfort. The pathophysiology involves autoimmune stimulation of orbital tissue, leading to increased vascularity, inflammation, and swelling. This process can cause mild discomfort and sensitivity.

**Why Each Wrong Option is Incorrect**
**Option A:**
This is incorrect because lid lag is indeed an early symptom, but it's not the earliest. Lid lag typically becomes more pronounced as the condition progresses.

**Option B:**
This is incorrect because exophthalmos is a more advanced sign of thyroid ophthalmopathy, often accompanied by other noticeable symptoms like eyelid retraction and eye discomfort.

**Option C:**
This is incorrect because eyelid retraction can occur early in the course of the disease but is not the initial symptom.

**Clinical Pearl / High-Yield Fact**
Recall that the earliest symptoms of thyroid ophthalmopathy are often mild and non-specific, including mild eye discomfort and sensitivity to light. These symptoms can be easily overlooked but are crucial for early diagnosis.

**Correct Answer:** A. Lid lag.