Final answer:
The ICD-10 code for a patient who has tested positive for HIV and has developed an HIV-related illness is B20. This code reflects an advanced stage of HIV infection, corresponding with AIDS and the presence of certain opportunistic illnesses.
Step-by-step explanation:
The ICD-10 code to be assigned to a patient who has had a positive serology or culture and has developed an HIV-related illness is D) B20. Code B20 indicates that a person has Human Immunodeficiency Virus (HIV) disease with specified conditions. If a patient has been tested positive for HIV and developed an HIV-related illness, it means the virus has progressed to a more advanced stage, which is characterized by a lower CD4 T-cell count and the presence of opportunistic illnesses or diseases directly related to the HIV infection.
For patients who test positive for HIV and have CD4 T-cell counts above 200 cells/μL without any apparent illness, like the case where CD4 count was 700/ μL, the infection would be in Stage 1 (A. 1) according to the provided context. This stage indicates a relatively preserved immune system and a lower risk of opportunistic infections. In contrast, Stage 3 is when AIDS is diagnosed either due to CD4 counts dropping below 200 cells/μL or the development of certain opportunistic illnesses, which corresponds with the use of the B20 code.