Primary immunodeficiency with rheumatic manifestations — formative SAQs

SAQ 1 (10)

A six-year-old girl presents with a three-month symmetric polyarthritis of the wrists and knees that has been labelled juvenile idiopathic arthritis. Over the same period she has had two pneumonias, chronic diarrhoea, and bruising from a platelet count of 40 × 10⁹/L. Her total IgG is well below the age-adjusted reference range. Her pneumococcal vaccine response is poor. [7] [8]

a) What is the most likely underlying diagnosis, and why does the combination of arthritis with recurrent infection, chronic diarrhoea, and autoimmune thrombocytopenia point to it rather than to ordinary juvenile idiopathic arthritis? (3 marks) [3]

b) Justify why immunoglobulin replacement alone is unlikely to control her arthritis and thrombocytopenia, and outline the principle that should govern their management. (3 marks) [7]

c) Outline the further investigations needed to characterise the cellular defect and to exclude secondary causes before committing to a lifelong label. (2 marks) [1]

d) Discuss the principles of long-term management, including the hazard of starting a biologic before the immune screen is complete, lung surveillance, and the disposition between immunology and rheumatology. (2 marks) [3]