**Question:** A 63-year-old woman experiences a burning sensation in her hands and feet. Two months ago, she had an episode of swelling with tenderness in the right leg, followed by dyspnea and right-sided chest pain. On physical examination, the spleen and liver now appear to be enlarged. CBC shows hemoglobin, 13.3 g/dL; hematocrit, 40.1%; MCV, 91 mm3; platelet count, 657,000/mm3; and WBC count, 17,400/mm3. The peripheral blood smear shows abnormally large platelets. Which of the following is the most likely diagnosis?

A. Polycythemia vera
B. Essential thrombocythemia
C. Myelofibrosis
D. Acquired thrombocytosis

**Correct Answer:** B. Essential thrombocythemia

**Core Concept:** Essential thrombocythemia (ET) is a type of myeloproliferative disorder characterized by excessive production of platelets, leading to an increased risk of thrombosis and bleeding complications.

**Why the Correct Answer is Right:** The correct answer is Essential Thrombocythemia (ET) due to the following reasons:

1. The patient's symptoms and clinical presentation include a history of swelling with tenderness in the right leg, dyspnea, and right-sided chest pain, which is consistent with the clinical features of ET.
2. The peripheral blood smear shows abnormally large platelets, which is a key feature of ET.
3. The patient's blood cell counts reveal elevated platelet count (657,000/mm3), confirming the diagnosis.
4. The patient has a normal hemoglobin level (13.3 g/dL), normal white blood cell count (17,400/mm3), and elevated hematocrit (40.1%).

**Why Other Options are Incorrect:**

A. Polycythemia vera (PV) is another type of myeloproliferative disorder, but it is characterized by elevated hemoglobin and red blood cell count rather than thrombocytosis (excessive platelet production). Therefore, PV is not the correct diagnosis.

C. Myelofibrosis (MF) is a different type of myeloproliferative disorder characterized by fibrosis of the bone marrow and elevated hemoglobin and red blood cell count. MF is also incorrect in this case.

D. Acquired thrombocytosis (elevated platelet count) can be caused by various etiologies, not just ET. Therefore, the elevated platelet count alone is not sufficient to diagnose ET.

**Clinical Pearls:**

1. ET is a clonal disorder, meaning it results from the clonal proliferation of abnormal megakaryocytes (platelet-forming cells) in the bone marrow.
2. ET is a myeloproliferative disorder that can lead to thrombosis (blood clots) and bleeding complications due to excessive platelet production.
3. The treatment of ET primarily aims at reducing platelet count to prevent complications, usually through hydroxyure