**Core Concept**
The classification of psychiatric illness is a crucial aspect of psychiatric diagnosis and treatment. Currently, the International Classification of Diseases (ICD) and the Diagnostic and Statistical Manual of Mental Disorders (DSM) are the two primary systems used to categorize psychiatric conditions. However, in 2009, the American Psychiatric Association (APA) released the DSM-5, which introduced significant changes to the classification system.

**Why the Correct Answer is Right**
The DSM-5 uses a categorical approach to classify psychiatric disorders, with a focus on symptom-based criteria and dimensional assessment. This system allows for more accurate diagnosis and treatment planning. The DSM-5 also introduced a new framework for personality disorders and eliminated subtypes for several disorders. The APA's decision to release the DSM-5 marked a significant shift in the way psychiatric illnesses are classified.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the ICD system, while widely used, is not the primary classification system used in the United States. The DSM-5 is the dominant classification system in American psychiatry.
**Option B:** This option is incorrect because the DSM-5 is a categorical system, not a dimensional one. While the DSM-5 does incorporate dimensional assessment, its primary approach is categorical.
**Option C:** This option is incorrect because the APA released the DSM-5 in 2013, not in 2009. The DSM-5 was published on May 18, 2013.

**Clinical Pearl / High-Yield Fact**
The DSM-5's use of a categorical approach to classification has been a topic of debate among psychiatrists. However, the new system has been widely adopted and is currently the standard for psychiatric diagnosis in the United States.

**Correct Answer: D. DSM-5**