Final answer:
The viral phenotype switch to the lymphocyte-tropic HIV variant leads to the progression from HIV to AIDS by causing a rapid decline in CD4 T-cell count and weakening the immune system.
Step-by-step explanation:
When about 50% of HIV cases undergo viral phenotype switches to the lymphocyte-tropic HIV variant late in infection, it leads to the serious deterioration of the immune system and progression to AIDS (Acquired Immunodeficiency Syndrome).
This occurs when the CD4 T-cell count drops below 200 cells/μl, which is one of the criteria for an AIDS diagnosis. The lymphocyte-tropic variant of HIV is more cytopathic and is associated with the rapid depletion of T-cells, further weakening the patient's immune response, and heralding the onset of stage 3, where opportunistic infections and rare cancers may develop due to the compromised immune system.