MRCP Part 1 Infectious Diseases and Immunology 2 — Questions and Answers
Question 1: A 35-year-old woman presents with fever, arthralgia, and a malar (butterfly) rash. Blood tests show pancytopaenia, raised anti-dsDNA antibodies, and low complement C3 and C4. What is the most likely diagnosis?
- Systemic lupus erythematosus (SLE) (Correct answer)
- Rheumatoid arthritis
- Dermatomyositis
- Systemic sclerosis
Correct answer: Systemic lupus erythematosus (SLE)
SLE characteristically presents in young women with multi-system involvement including malar rash, arthralgia, cytopaenias, and renal disease. Anti-dsDNA antibodies are highly specific for SLE and correlate with disease activity, particularly lupus nephritis. Low complement levels indicate active immune complex consumption.
Question 2: A renal transplant recipient on ciclosporin, mycophenolate, and prednisolone develops fever and a CT chest shows ground-glass opacification. Bronchoalveolar lavage with silver staining reveals cysts. What is the causative organism?
- Pneumocystis jirovecii (Correct answer)
- Mycobacterium tuberculosis
- Aspergillus fumigatus
- Cytomegalovirus
Correct answer: Pneumocystis jirovecii
Pneumocystis jirovecii pneumonia (PJP) typically presents in immunosuppressed patients with progressive dyspnoea, dry cough, and fever. CT shows bilateral ground-glass opacification. Diagnosis is confirmed by identifying cysts on silver staining of BAL fluid. Treatment is high-dose co-trimoxazole, with adjunctive steroids if hypoxic.
Question 3: A 40-year-old man presents with recurrent sinopulmonary infections and chronic diarrhoea. Serum immunoglobulins show very low IgG, IgA, and IgM. B-cell numbers are markedly reduced. What is the most likely diagnosis?
- Common variable immunodeficiency (CVID) (Correct answer)
- Selective IgA deficiency
- X-linked agammaglobulinaemia
- DiGeorge syndrome
Correct answer: Common variable immunodeficiency (CVID)
Common variable immunodeficiency (CVID) is the most common symptomatic primary antibody deficiency in adults, presenting with recurrent infections and low immunoglobulin levels. Unlike X-linked agammaglobulinaemia (which presents in infancy with absent B cells), CVID typically presents in the second or third decade with reduced but not absent B cells.
Question 4: A patient returns from South-East Asia with cyclical fevers every 48 hours, rigors, and splenomegaly. Blood film shows ring forms and banana-shaped gametocytes within red blood cells. Which species of Plasmodium is responsible?
- Plasmodium falciparum (Correct answer)
- Plasmodium vivax
- Plasmodium ovale
- Plasmodium malariae
Correct answer: Plasmodium falciparum
Banana-shaped (crescent) gametocytes are pathognomonic of Plasmodium falciparum. While P. falciparum classically causes irregular fevers rather than strict 48-hour cycles, it can produce tertian periodicity. P. falciparum is the most dangerous species, capable of causing cerebral malaria, severe anaemia, and multi-organ failure.
Question 5: A 25-year-old man presents with a 2-week history of productive cough, night sweats, and weight loss after returning from India. Sputum AFB smear is positive. He lives with his partner and 2-year-old child. What is the most important public health action?
- Contact tracing and screening of household contacts (Correct answer)
- Isolating the patient in a negative pressure room permanently
- Vaccinating all contacts with BCG immediately
- Notifying the WHO directly
Correct answer: Contact tracing and screening of household contacts
Active pulmonary TB is notifiable, and contact tracing is the most important public health intervention. Household contacts, especially children under 5, are at highest risk. Contacts should be screened with symptom assessment, chest X-ray, and interferon-gamma release assay (IGRA). The child may need chemoprophylaxis regardless of screening results due to young age.
Question 6: Which investigation is used to screen for latent tuberculosis prior to starting biologic therapy, and how does it work?
- Interferon-gamma release assay (IGRA) — measures T-cell IFN-gamma release in response to TB antigens (Correct answer)
- Mantoux test — measures B-cell antibody response to tuberculin
- Sputum culture — grows TB from respiratory secretions
- Chest X-ray — identifies active pulmonary disease
Correct answer: Interferon-gamma release assay (IGRA) — measures T-cell IFN-gamma release in response to TB antigens
IGRA (e.g., QuantiFERON-TB Gold) measures interferon-gamma release from T-cells sensitised to M. tuberculosis-specific antigens (ESAT-6 and CFP-10). Unlike the Mantoux test, IGRA is not affected by prior BCG vaccination, making it preferred for screening in BCG-vaccinated populations such as the UK.
MRCP Part 1 Infectious Diseases and Immunology 2
A 35-year-old woman presents with fever, arthralgia, and a malar (butterfly) rash.
Blood tests show pancytopaenia, raised anti-dsDNA antibodies, and low complement C3 and C4.
What is the most likely diagnosis?