RBC count is low in anaemia and

The correct option is Leukemia.

To understand why this is correct, let's break down the physiology of red blood cells (RBCs) and how certain medical conditions affect their count in the body:
1. Anemia: This is a condition characterized by a deficiency in the number of healthy red blood cells or the amount of hemoglobin they contain, which leads to reduced oxygen flow to the body's organs.
2. Leukemia: Leukemia is a type of cancer that affects the blood and bone marrow, where blood cells are produced. In leukemia, the bone marrow starts producing an abnormal and excessive number of white blood cells (leukocytes) that do not function properly.
3. The bone marrow connection: Because the bone marrow is overcrowded with these rapidly multiplying, abnormal white blood cells, it lacks the space and resources to produce normal levels of other blood components. Consequently, the production of healthy red blood cells (RBCs) and platelets is severely suppressed.
4. Resulting Low RBC Count: Due to this crowding out effect in the bone marrow, patients with leukemia commonly develop a low RBC count (anemia) alongside their high white blood cell count.

Comparing with other options:
- Myxoedema: This refers to severe hypothyroidism (underactive thyroid gland), which can cause a general slowing down of bodily functions but is not primarily defined by a rapid disruption of bone marrow RBC production like leukemia.
- Influenza & Typhoid: These are infectious diseases (viral and bacterial, respectively) that trigger immune responses but do not characteristically cause a primary, severe drop in RBC counts in the same pathophysiological manner as leukemia's bone marrow crowding.