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]

SAQ 2 (10)

An eight-year-old boy has a granulomatous colitis with perianal disease that is indistinguishable from Crohn disease on biopsy, but he has also had a staphylococcal liver abscess and a Serratia pneumonia. Separately, a five-year-old boy presents with systemic lupus erythematosus, nephritis, a family history of lupus and meningococcal disease, and a total haemolytic complement (CH50) that is persistently absent when checked well. [4] [9]

a) For the first child, name the most likely underlying immune defect and the single confirmatory test. Explain why the organism cluster (Staphylococcus, Serratia) and the granulomatous colitis point to it. (3 marks) [4]

b) Explain why treating this child's colitis with immunosuppression before the confirmatory test is dangerous, and outline the principles of management including prophylaxis and the role of haematopoietic stem cell transplantation. (3 marks) [5]

c) For the second child, identify the likely category of diagnosis, explain the mechanism by which classical complement deficiency produces childhood-onset lupus, and justify why the CH50 must be checked when the child is well rather than during a flare. (2 marks) [9] [10]

d) Describe the additional prevention that classical complement deficiency requires beyond standard lupus therapy, and the family-screening implications. (2 marks) [9]