**Core Concept**
The patient's symptoms and physical examination findings suggest a diagnosis of Essential Thrombocythemia (ET), a myeloproliferative neoplasm characterized by an excessive production of platelets. ET is often associated with splenomegaly, easy bruising, and an elevated platelet count.

**Why the Correct Answer is Right**
In ET, the platelet count is elevated due to an overproduction of platelets by the bone marrow. The leucocyte alkaline phosphatase score is typically normal in ET, which helps to differentiate it from other myeloproliferative neoplasms such as Chronic Myeloid Leukemia (CML). The drug of choice for the treatment of ET is anagrelide, which works by inhibiting platelet production in the bone marrow. Anagrelide is a selective platelet inhibitor that reduces the platelet count without affecting the leucocyte or erythrocyte counts.

**Why Each Wrong Option is Incorrect**
**Option A:** Hydroxyurea is a cytotoxic agent that can be used to reduce the platelet count in ET, but it is not the first-line treatment and is associated with more side effects compared to anagrelide.

**Option B:** Radioactive phosphorus is a treatment option for ET, but it is typically reserved for patients who are resistant to anagrelide or other treatments.

**Option D:** Interferon alpha is used to treat other myeloproliferative neoplasms such as CML and Polycythemia Vera, but it is not the preferred treatment for ET.

**Clinical Pearl / High-Yield Fact**
Anagrelide is a selective platelet inhibitor that is often used as a first-line treatment for ET due to its efficacy and relatively mild side effect profile. It is essential to monitor the platelet count regularly when using anagrelide to avoid thrombocytopenia.

**Correct Answer:** C. Anagrelide