**Core Concept**
The question pertains to a case of progressive ptosis with diplopia, suggesting a condition involving the cranial nerves or their nuclei. The underlying principle being tested is the anatomy and function of the third cranial nerve (oculomotor nerve), which is responsible for controlling the movement of the eyeball.

**Why the Correct Answer is Right**
The correct diagnosis in this case is a third nerve palsy, specifically a **myasthenic third nerve palsy**. This condition is characterized by progressive ptosis and diplopia due to muscle weakness, which worsens with increased activity. The third cranial nerve controls the levator palpebrae superioris muscle, which elevates the eyelid, and the medial rectus muscle, which rotates the eye inward. In myasthenic third nerve palsy, the weakness of these muscles leads to progressive ptosis and diplopia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not explain the progressive worsening of ptosis and diplopia.
**Option B:** This option is incorrect because it does not involve the third cranial nerve, which is responsible for the symptoms described.
**Option C:** This option is incorrect because it is not a recognized condition involving the third cranial nerve.

**Clinical Pearl / High-Yield Fact**
Myasthenic third nerve palsy is a classic example of a "double crush" phenomenon, where two conditions (myasthenia gravis and third nerve palsy) coexist, leading to a complex clinical presentation. This highlights the importance of considering multiple diagnoses when evaluating a patient's symptoms.

**Correct Answer:** C. Myasthenic third nerve palsy.