**Question:** Drug producing Haematological side effect is:-

A. Hydroxyurea
B. Cytosine arabinoside
C. Thalidomide
D. 5-Fluorouracil

**Core Concept:**

The question is asking about drugs that can cause haematological side effects, specifically referring to the field of haematology. Haematological side effects can be classified into three main categories: blood cell depletion, blood cell dyscrasias, and overproduction of blood cells. These side effects are often associated with the use of certain drugs, particularly those used in cancer treatment and management.

**Why the Correct Answer is Right:**

**Option C: Thalidomide** is a drug that belongs to the category of anti-angiogenic agents, used primarily for the treatment of multiple myeloma and leprosy. Amongst its side effects, Thalidomide causes aplastic anaemia, which is a blood cell depletion disorder. This side effect is caused by direct cytotoxicity of the drug towards hematopoietic stem cells, leading to a decrease in the production of red blood cells, white blood cells, and platelets.

**Why Each Wrong Option is Incorrect:**

**Option A: Hydroxyurea** is an antineoplastic agent used for the treatment of chronic myelogenous leukemia, sickle cell disease, and polycythemia vera. While hydroxyurea does have haematological side effects, they are primarily related to its effects on red blood cells. Common side effects include a decrease in red blood cell count, which is a form of blood cell depletion.

**Option B: Cytosine arabinoside** is an antineoplastic agent used to treat acute leukemia. It is an anticancer drug that affects the DNA synthesis in rapidly dividing cells, including hematopoietic cells in the bone marrow. Cytosine arabinoside can cause aplastic anaemia, a blood cell depletion disorder as mentioned earlier.

**Option D: 5-Fluorouracil** is an antimetabolite used for the treatment of various cancers, including colorectal, pancreatic, and breast cancer. Its primary mode of action involves inhibiting DNA and RNA synthesis in rapidly dividing cells. Although 5-Fluorouracil does have haematological side effects, they are primarily related to the suppression of the bone marrow, leading to decreased production of all types of blood cells.

**Clinical Pearl:** Haematological side effects of chemotherapy drugs, including those mentioned above, often present as a decrease in the production of red blood cells (anaemia), white blood cells (leucopenia), and platelets (thrombocytopenia). These side effects are typically dose-dependent and resolve once the drug is discontinued or the dose is reduced. However, it is essential for clinicians to be aware of these side effects when prescribing chemotherapy drugs to monitor patients carefully and adjust treatment accordingly.