**Core Concept**
A stem cell disorder affecting all three cell lines (platelets, RBCs, and leucocytes) is a condition characterized by bone marrow failure. This is due to a defect in the hematopoietic stem cells, which are responsible for producing all blood cells.

**Why the Correct Answer is Right**
The condition described is Fanconi anemia (FA), a rare genetic disorder caused by mutations in the FANCA, FANCC, FANCD1 (BRCA2), FANCD2, FANCE, FANCI, FANCL, FANCM, FANCO, and FANCP genes. These genes encode proteins involved in the Fanconi anemia pathway, which is essential for the repair of interstrand DNA crosslinks. Defects in this pathway lead to genomic instability, chromosomal breakage, and ultimately, bone marrow failure. FA is characterized by a triad of pancytopenia (low counts of platelets, RBCs, and leucocytes), congenital abnormalities, and an increased risk of malignancies.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a condition affecting all three cell lines. For example, idiopathic thrombocytopenic purpura (ITP) primarily affects platelets.
* **Option B:** This option is incorrect because it does not accurately describe a condition affecting all three cell lines. For example, aplastic anemia primarily affects the bone marrow's ability to produce blood cells, but it is not a stem cell disorder affecting all three cell lines.
* **Option C:** This option is incorrect because it does not specify a condition affecting all three cell lines. For example, paroxysmal nocturnal hemoglobinuria (PNH) primarily affects RBCs.

**Clinical Pearl / High-Yield Fact**
Fanconi anemia is a rare genetic disorder with a high risk of malignancies, particularly acute myeloid leukemia (AML) and squamous cell carcinomas. Patients with FA often require bone marrow transplantation for treatment.

**Correct Answer:** C.