**Core Concept**
Exposure and Response Prevention (ERP) therapy is a cognitive-behavioral therapy used to treat Obsessive-Compulsive Disorder (OCD). It involves helping patients confront situations that trigger obsessive thoughts while preventing them from engaging in compulsive behaviors. The effectiveness of ERP therapy can be influenced by various factors.

**Why the Correct Answer is Right**
A poor prognostic factor in OCD is the presence of **comorbid tic disorders**. Tics are sudden, repetitive movements or sounds that can be a hallmark of conditions like Tourette's syndrome. Research suggests that individuals with OCD and comorbid tic disorders tend to have a poorer response to ERP therapy. This may be due to the complex interplay between OCD symptoms and tic-related behaviors, making it more challenging to implement effective therapeutic strategies.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Co-morbid depression** is not a poor prognostic factor in OCD. While depression can certainly impact treatment outcomes, it does not specifically undermine the effectiveness of ERP therapy.
* **Option B:** **Age of onset** is not a direct prognostic factor for ERP therapy. Early onset of OCD may be associated with a more severe course, but it does not necessarily predict treatment response.
* **Option C:** **Family history of OCD** is not a poor prognostic factor. A family history of OCD may actually be a risk factor for developing OCD, but it does not necessarily affect treatment outcomes.

**Clinical Pearl / High-Yield Fact**
When treating OCD with ERP therapy, it's essential to consider the presence of comorbid tic disorders. These conditions can complicate treatment and require a more tailored approach.

**Correct Answer: C. Family history of OCD**