**Core Concept**
Progressive Multifocal Leukoencephalopathy (PML) is a rare and often fatal viral disease caused by the JC virus. PML affects the white matter of the brain, leading to demyelination and neurological symptoms. The disease is characterized by an imbalance between the JC virus and the host's immune system.

**Why the Correct Answer is Right**
PML is characterized by the presence of oligodendrocytes that are infected with the JC virus. These infected cells are typically positive for CD 15 (also known as CD1c), but they are usually negative for CD 34. This is because CD 34 is a marker of hematopoietic stem cells and progenitor cells, which are not typically present in the brain tissue affected by PML. The co-expression of CD 15 and CD 34 in the same cell is not a characteristic feature of PML.

**Why Each Wrong Option is Incorrect**
**Option A:** Retinoic acid is not a standard treatment for PML. While it has been investigated as a potential therapeutic agent, its efficacy is still unclear.
**Option B:** The 15/17 translocation is actually associated with acute promyelocytic leukemia (APL), a type of acute myeloid leukemia. This translocation is not related to PML.
**Option D:** Disseminated intravascular coagulation (DIVC) is not a characteristic feature of PML. While coagulopathy can occur in patients with PML, it is not a defining feature of the disease.

**Clinical Pearl / High-Yield Fact**
PML is a disease of immunocompromised patients, particularly those with HIV/AIDS or taking immunosuppressive medications. It is essential to maintain a high index of suspicion for PML in patients with unexplained neurological symptoms and a history of immunosuppression.

**✓ Correct Answer:** C. CD 15/34 both seen in same cell