FRCPath Haem Part 1 MCQs-Morphology 480
A 46-year-old man presents with recurrent flushing, abdominal discomfort, pruritus and episodes of unexplained hypotension. He has mild hepatosplenomegaly.
FBC shows:
Hb: 108 g/L
WBC: 9.8 × 10⁹/L
Platelets: 165 × 10⁹/L
A bone marrow aspirate is haemodiluted. The trephine biopsy shows fibrosis, particularly around trabeculae. There is trabeculae thickening and distortion
Immunohistochemistry demonstrates strong expression of CD117. The cells show aberrant CD25 expression.
Which of the following is the single most likely diagnosis?
A. Mast cell activation syndrome
B. Systemic mastocytosis
C. Acute myeloid leukaemia
D. Chronic myeloid leukaemia
E. Systemic mastocytosis with associated neoplasm
Answer: B. Systemic mastocytosis
Explanation
This is classic for systemic mastocytosis, with several high-yield FRCPath clues:
Dense multifocal mast-cell aggregates in bone marrow
Spindle-shaped atypical mast cells
CD117 and tryptase positive
Aberrant CD25 expression
KIT D816V mutation
Markedly elevated serum tryptase
Symptoms caused by mast-cell mediator release — flushing, pruritus, abdominal symptoms and hypotension.
FRCPath Morphology Association
Systemic mastocytosis → thickened trabeculae + peritrabecular mast-cell aggregates + spindle-shaped mast cells.
Exam trap: Don't rely solely on the presence of an elevated tryptase. The bone marrow mast-cell aggregates and aberrant mast-cell phenotype, particularly CD25, are major diagnostic clues.
Another FRCPath-level association
If the question shows:
Spindle-shaped mast cells + dense aggregates + CD25/CD2 aberrancy + KIT D816V