**Core Concept**
Paroxysmal nocturnal haemoglobinuria (PNH) is a rare, acquired, life-threatening disease of the blood characterized by the destruction of red blood cells (hemolysis), bone marrow failure, and the possible presence of blood clots. The condition is caused by a mutation in the PIGA gene, leading to the deficiency of glycosylphosphatidylinositol (GPI) anchors on the surface of hematopoietic stem cells.

**Why the Correct Answer is Right**
Increased alkaline phosphatase is not typically associated with PNH. This enzyme is usually elevated in conditions involving bone growth or liver disease, such as Paget's disease, osteomalacia, or hepatitis. In contrast, PNH is characterized by bone marrow failure, which can manifest as pancytopenia (a reduction in the number of red and white blood cells and platelets).

**Why Each Wrong Option is Incorrect**
**Option A:** Haemosiderinuria (the presence of iron in the urine) is indeed a feature of PNH. When red blood cells are destroyed, the iron from the hemoglobin is released and excreted in the urine.

**Option B:** Pancytopenia is a common manifestation of PNH, resulting from the bone marrow failure associated with the disease. Patients may experience a reduction in the number of red and white blood cells and platelets.

**Option D:** Cellular marrow (indicative of bone marrow involvement) can be seen in PNH. The bone marrow may appear hypercellular due to the proliferation of abnormal stem cells.

**Clinical Pearl / High-Yield Fact**
PNH is often diagnosed when patients present with recurring episodes of hemolytic anemia, thrombosis, and bone marrow failure. The disease is associated with a high risk of thrombosis and can be challenging to diagnose due to its rarity and nonspecific symptoms.

**✓ Correct Answer: C. Increased alkaline phosphatase**