**Core Concept:**
Full blown Immunodeficiency syndrome refers to a severe and advanced stage of Human Immunodeficiency Virus (HIV) infection. HIV primarily targets and destroys CD4+ T cells, a type of white blood cell crucial for coordinating immune responses. As the infection progresses, the immune system weakens, increasing susceptibility to opportunistic infections and malignancies.

**Why the Correct Answer is Right:**
The correct answer is HIV, which stands for Human Immunodeficiency Virus. HIV is the causative agent for full blown Immunodeficiency syndrome, as it specifically targets and depletes CD4+ T cells, compromising the immune system.

**Why Each Wrong Option is Incorrect:**
A. Opportunistic infections and malignancies are consequences of HIV infection and not the virus itself. These are not the correct answer.
B. AIDS (Acquired Immunodeficiency Syndrome) is the clinical term for the advanced stage of HIV infection, not a virus.
C. This option refers to a different disease, Kaposi's sarcoma, which is a malignancy associated with HIV infection. It is not the correct answer for the virus causing full blown Immunodeficiency syndrome.
D. Acquired Immunodeficiency Syndrome (AIDS) is another clinical term for the advanced stage of HIV infection, similar to B, but not the virus itself.

**Clinical Pearl / High-Yield Fact:**
HIV triggers a cascade of events leading to a weakened immune system, making the individual susceptible to severe infections and cancers. This emphasizes the importance of early diagnosis, treatment, and prevention strategies to reduce the impact of AIDS.

**Correct Answer:**
HIV (Human Immunodeficiency Virus)

**Explanation:**
HIV is the correct answer as it is the virus responsible for causing full blown Immunodeficiency syndrome by specifically targeting CD4+ T cells, which are crucial for immune system function. The other options (opportunistic infections, AIDS, and Kaposi's sarcoma) are clinical manifestations or terms used to describe the advanced stages of HIV infection, not the virus itself.