**Core Concept**
Forced duction test is a clinical examination technique used to assess the mechanical function of the extraocular muscles, which control eye movements. This test helps in differentiating between mechanical and non-mechanical (e.g., neurogenic) causes of eye movement disorders.

**Why the Correct Answer is Right**
During a forced duction test, the examiner manually moves the eye in the direction of the suspected muscle's action, while the patient is under local anesthesia to prevent voluntary muscle contraction. If the movement is restricted, it indicates a mechanical block or paralysis of the muscle, which could be due to trauma, tumor, or other causes. The test relies on the principle that the examiner can move the eye past the point where voluntary movement is restricted, if the mechanical restriction is present.

**Why Each Wrong Option is Incorrect**
**Option A:** Strabismus - While strabismus is a condition that affects eye alignment, the forced duction test is not primarily used to diagnose strabismus, but rather to assess the mechanical function of the extraocular muscles.

**Option B:** Diplopia - Diplopia (double vision) is a symptom that can result from a variety of causes, including mechanical restrictions of the extraocular muscles. However, the forced duction test is a diagnostic tool, not a treatment for diplopia.

**Option C:** Ptosis - Ptosis (drooping of the eyelid) is a condition that affects the levator palpebrae superioris muscle, which controls eyelid opening. The forced duction test is not applicable to ptosis.

**Option D:** Nystagmus - Nystagmus is an involuntary movement of the eyes that can be caused by a variety of factors, including neurological disorders. The forced duction test is not used to diagnose nystagmus.

**Clinical Pearl / High-Yield Fact**
The forced duction test is a useful diagnostic tool in cases of suspected mechanical restriction of the extraocular muscles, such as in thyroid eye disease or orbital trauma. It helps in differentiating between mechanical and non-mechanical causes of eye movement disorders.

**Correct Answer:** C. Mechanical restriction of the extraocular muscles.