**Core Concept**
Polycythemia rubra vera (PRV) is a myeloproliferative neoplasm characterized by the excessive production of red blood cells, white blood cells, and platelets. This condition is often associated with mutations in the JAK2 gene, leading to the activation of the JAK/STAT signaling pathway and subsequent proliferation of hematopoietic cells.

**Why the Correct Answer is Right**
The Leukocyte Alkaline Phosphatase (LAP) score is typically elevated in PRV due to the increased production of leukocytes. In contrast, a decreased LAP score is not characteristic of PRV. This is because the increased LAP score in PRV is due to the high turnover of leukocytes, which leads to the release of alkaline phosphatase into the bloodstream. Therefore, an elevated LAP score is often used as a diagnostic marker for PRV.

**Why Each Wrong Option is Incorrect**
**Option A:** Increased Vit B12 binding capacity is actually seen in PRV, as the increased red blood cell mass leads to an increase in vitamin B12 binding capacity. This is because vitamin B12 is stored in the liver and bound to transcobalamin I and III, which are released into the bloodstream in response to increased red blood cell turnover.

**Option C:** Leucocytosis, or an elevated white blood cell count, is a common feature of PRV due to the excessive production of leukocytes.

**Option D:** Increased platelets are also seen in PRV, as the JAK2 mutation leads to the activation of the JAK/STAT pathway and subsequent proliferation of megakaryocytes.

**Clinical Pearl / High-Yield Fact**
In PRV, the increased red blood cell mass can lead to an increased risk of thrombosis and other vascular complications. Therefore, it is essential to monitor patients with PRV for signs of hyperviscosity and to consider therapeutic interventions such as phlebotomy to reduce the risk of complications.

**✓ Correct Answer: B. Decreased LAP Score**