According to ICD-10, which letter is used to denote mental illness?

**Core Concept**
The ICD-10 (International Classification of Diseases, 10th Revision) is a widely used system for classifying diseases and health problems. Mental and behavioral disorders are categorized using specific codes to facilitate accurate diagnosis and treatment planning.

**Why the Correct Answer is Right**
Mental illnesses are classified under the category of F codes in ICD-10, which includes a range of conditions such as depressive disorders, anxiety disorders, and schizophrenia. These codes enable healthcare professionals to accurately diagnose and document mental health conditions, facilitating communication and treatment among healthcare providers. The F codes are further divided into subcategories, such as F00-F09 for organic, including symptomatic, mental disorders and F10-F19 for mental and behavioral disorders due to psychoactive substance use.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because there is no specific letter or code that denotes mental illness in ICD-10.
* **Option B:** This option is incorrect because it does not accurately represent the classification of mental illnesses in ICD-10.
* **Option D:** This option is incorrect because there is no specific letter or code that denotes mental illness in ICD-10.

**Clinical Pearl / High-Yield Fact**
When coding mental illnesses in ICD-10, it is essential to use the correct F codes to ensure accurate diagnosis and treatment planning. Familiarity with the ICD-10 coding system is crucial for healthcare professionals to provide high-quality care and communicate effectively with patients and other healthcare providers.

**Correct Answer:** F. F codes.