## **Core Concept**
Myasthenia gravis (MG) is an autoimmune disorder characterized by weakness and rapid fatigue of the muscles under voluntary control. It is caused by a breakdown in the normal communication between nerves and muscles due to antibodies blocking or destroying nicotinic acetylcholine receptors (AChR) at the neuromuscular junction.

## **Why the Correct Answer is Right**
The correct answer, **D. Bulbar muscles**, or more specifically muscles supplied by the **cranial nerves**, are indeed affected in myasthenia gravis. However, extraocular muscles (controlled by cranial nerves III, IV, and VI) and bulbar muscles are often involved early and significantly. Despite this, some muscle groups are less affected or have a different pattern of involvement. The question seems to focus on the relative sparing or pattern of muscle involvement.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Proximal limb muscles are commonly affected in myasthenia gravis. These muscles are among the first to show symptoms, making this option incorrect as the muscles here are not the least affected.
- **Option B:** Distal limb muscles are generally less affected initially compared to proximal muscles but can become involved as the disease progresses. However, they are not the least affected when considering the typical presentation and progression.
- **Option C:** Respiratory muscles can become involved and are a significant concern in myasthenia gravis, particularly in myasthenic crises. Their involvement can be life-threatening, indicating they are not the least affected.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that in myasthenia gravis, **extraocular muscles** are often involved early, but **pupillary reflexes remain intact** because the autonomic (involuntary) nervous system is not affected by the disease process. This can help differentiate MG from other causes of diplopia.

## **Correct Answer:** . Bulbar muscles