**Core Concept:** OCD (Obsessive-Compulsive Disorder) is a psychiatric disorder characterized by recurrent and persistent thoughts (obsessions) and/or urges (compulsions) that cause significant distress or impairment in daily functioning. Exposure and Response Prevention (ERP) is a behavioral therapy that involves exposing the patient to their obsessional trigger and preventing them from engaging in compulsive behaviors to reduce distress.

**Why the Correct Answer is Right:** Type C of OCD is referred to as "pure" or "pure-pure" OCD. In this type, the patient experiences only obsessional thoughts without compulsive behaviors. ERP therapy is effective in treating OCD by reducing both obsessions and compulsions. Since the patient with pure OCD has no compulsive behaviors to prevent, ERP therapy is less effective in treating their symptoms.

**Why Each Wrong Option is Incorrect:**

A. Exposure and response prevention therapy is effective in treating OCD, which rules out option A.
B. Pure-pure OCD involves only obsessional thoughts and not compulsive behaviors, making ERP therapy less effective. This rules out option B.
D. Pure-pure OCD is a specific type of OCD with no compulsive behaviors, making ERP therapy less effective. This rules out option D.

**Why Each Wrong Option is Incorrect:**

C. While pure-pure OCD involves only obsessional thoughts, the correct answer refers to the therapy response to this type of OCD. The presence of obsessional thoughts does not directly rule out ERP therapy, so option C is incorrect as well. However, the core reason for choosing option C is the focus on ERP therapy response.

**Clinical Pearl:** Pure-pure OCD has a poor prognosis due to the lack of compulsive behaviors in response to ERP therapy. This type of OCD is often more challenging to treat as it lacks the target for ERP therapy.