Neural explanations to OCD evaluation

Supporting evidence: 

Some antidepressants may work purely on the serotonin system, increasing levels of this neurotransmitter. Such drugs are effective in reducing OCD symptoms and this suggests that the serotonin system is involved in OCD. Biological processes can also cause the symptoms in those conditions which might be responsible for OCD. 

Not clear what neural mechanisms are involved: 

Some studies have shown that neural systems are the same systems that function abnormally in OCD. However, research has also identified other brain systems that may sometimes be involved but no system has been found that always plays a role in OCD. Therefore, we can’t claim to understand the neural mechanisms involved in OCD. 

Evidence suggesting that OCD isn’t caused by neural mechanisms: 

There is evidence suggesting many neurotransmitters and structures of the brain don’t function normally for people who have OCD. But it doesn’t mean that abnormal functioning in the brain caused OCD but rather they can be a result of OCD not a cause.

Serotonin OCD link might be comorbid with depression: 

Many people who have OCD might be depressed - which might disrupt the serotonin system. Which leaves us with a logical problem when it comes to the serotonin system as a possible basis for OCD. Some patients of OCD with disrupted serotonin systems might also suffer from depression as well.