What is the first character type in valid ICD-10-CM three character category codes?

In valid ICD-10-CM three-character category codes, the first character is always a letter. The letter serves as an important identifier that categorizes the diagnosis. Each letter corresponds to a specific block of diagnoses, which helps in organizing and classifying medical conditions in a meaningful way.

For example, categories starting with the letter "A" are related to infectious and parasitic diseases, while those starting with "E" pertain to endocrine, nutritional, and metabolic diseases. This structure allows healthcare providers and coders to easily locate and utilize the appropriate code for a patient's diagnosis.

In contrast, the other character types presented in the choices do not serve as the first character in these codes. Therefore, understanding that the first character is always a letter is crucial for correctly interpreting and using ICD-10-CM coding.