**Core Concept**
Tabes dorsalis is a late manifestation of neurosyphilis, characterized by degeneration of the posterior columns of the spinal cord. This condition leads to various neurological symptoms, including ophthalmoplegia, due to involvement of the cranial nerves. The oculomotor nerve (III nerve) is responsible for controlling eye movements, including adduction, abduction, and rotation.

**Why the Correct Answer is Right**
The oculomotor nerve (III nerve) is the most commonly affected cranial nerve in tabes dorsalis. This is because the disease process involves the degeneration of the posterior columns of the spinal cord, which can lead to the involvement of the nerve fibers that arise from the brainstem, including those of the oculomotor nerve. The oculomotor nerve is responsible for controlling eye movements, including adduction, abduction, and rotation, and its involvement can lead to ophthalmoplegia.

**Why Each Wrong Option is Incorrect**
**Option A:** The trochlear nerve (IV nerve) is not the most commonly affected cranial nerve in tabes dorsalis. While it can be affected, the oculomotor nerve is more commonly involved due to its anatomical location and the disease process.
**Option B:** The trochlear nerve (IV nerve) is responsible for controlling the superior oblique muscle, which is involved in eye movements, but it is not the most commonly affected cranial nerve in tabes dorsalis.
**Option C:** The abducens nerve (VI nerve) is responsible for controlling the lateral rectus muscle, which is involved in eye movements, but it is not the most commonly affected cranial nerve in tabes dorsalis.
**Option D:** Total ophthalmoplegia is a condition where there is complete paralysis of all extraocular muscles, but it is not a specific cranial nerve palsy.

**Clinical Pearl / High-Yield Fact**
In tabes dorsalis, the oculomotor nerve palsy is often associated with other neurological symptoms, such as ataxia, dysarthria, and loss of vibration sense. A high index of suspicion for neurosyphilis should be maintained in patients with tabes dorsalis, and prompt treatment with antibiotics is essential to prevent further neurological deterioration.

**✓ Correct Answer: A. III nerve**