**Core Concept**
Actinomycosis is a rare chronic bacterial infection caused by Actinomyces israelii, often presenting as a destructive granulomatous disease. In spinal actinomycosis, the infection tends to spread along fascial planes and can erode through various structures.

**Why the Correct Answer is Right**
The abscess in spinal actinomycosis typically erodes towards the skin due to its propensity to spread along fascial planes and the body's natural tendency to attempt to "drain" abscesses through the surface. This is because Actinomyces israelii is a normal commensal organism in the oral cavity and gastrointestinal tract, and its infection often leads to a localized inflammatory response that tries to expel the pathogen. The skin being the most accessible route for drainage, the abscess tends to erode towards it.

**Why Each Wrong Option is Incorrect**
* **Option A:** While intervertebral discs can be involved in the infection, they are not the primary site of abscess erosion. Actinomycosis tends to cause more destructive lesions in the surrounding bone and soft tissues rather than the disc itself.
* **Option B:** Extension into the pleural cavity is not a typical feature of spinal actinomycosis. This option is more characteristic of other types of spinal infections, such as tuberculosis or pyogenic infections.
* **Option C:** Retroperitoneal space involvement is not a common feature of spinal actinomycosis. While the infection can spread to surrounding soft tissues, it tends to follow a more superficial course rather than deep into the retroperitoneal space.

**Clinical Pearl / High-Yield Fact**
Actinomycosis is a "sulfur granule" disease, characterized by the presence of yellowish, gritty granules in the pus that contain the bacteria. This unique feature can aid in the diagnosis of actinomycosis, especially in cases where the infection is not responding to standard antibiotic therapy.

**✓ Correct Answer: D. Towards the skin**