**Core Concept**
A retrobulbar anesthetic block is a regional anesthesia technique used to anesthetize the eye and surrounding structures. It involves injecting local anesthetic into the retrobulbar space, which is located behind the eyeball. This technique is commonly used for ophthalmic surgeries. The effectiveness of the block depends on the spread of the local anesthetic to the surrounding muscles.

**Why the Correct Answer is Right**
The last muscle to be rendered akinetic (paralyzed) with a retrobulbar anesthetic block is the inferior oblique muscle. This is because the inferior oblique muscle is the most posterior and medial of the extraocular muscles, and the local anesthetic may take longer to spread to this area due to its anatomical location. The inferior oblique muscle is responsible for elevating the eye, and its paralysis can affect the patient's ability to look upward.

**Why Each Wrong Option is Incorrect**
* **Option A:** The superior rectus muscle is typically one of the first muscles to be affected by a retrobulbar block due to its anterior location.
* **Option B:** The lateral rectus muscle is responsible for outward gaze and is often affected early in the block due to its proximity to the injection site.
* **Option D:** The medial rectus muscle is responsible for inward gaze and is typically affected early in the block due to its proximity to the injection site.

**Clinical Pearl / High-Yield Fact**
It is essential to carefully assess the patient's eye movements after a retrobulbar block to ensure that the block is effective and to avoid injury to the eye. The patient's ability to move their eye upward can indicate whether the inferior oblique muscle is adequately paralyzed.

**Correct Answer:** C.