Autoimmunity and Immunodeficiency Questions and Answers

Answer: D

The autoimmune disease is myasthenia gravis.

In myasthenia gravis the body makes antibodies against the acetylcholine receptors of the neuromuscular junction (and in some patients against MuSK). The antibodies block and destroy the receptors, so nerve impulses fail to excite muscle fully, producing muscle weakness that worsens with use.

Parkinson's disease is a degeneration of dopamine-producing neurons of the substantia nigra, and Alzheimer's is a neurodegenerative disease with amyloid plaques and tau tangles; neither is classed as autoimmune. Spasticity is a symptom, increased muscle tone from upper motor neuron damage, not a disease mechanism.

Answer: B

The most likely diagnosis is severe combined immunodeficiency (SCID).

SCID is a defect of both T-cell and B-cell function, so the child has very low counts of both and suffers repeated bacterial infections, along with viral and fungal ones. The thymus fails to develop properly, which explains the absent thymic shadow on a chest film.

HIV infection is acquired and does not present as absent B cells in a young child. Chronic granulomatous disease is a defect of phagocyte killing, with normal lymphocyte counts. DiGeorge syndrome also gives an absent thymus, but it affects the T-cell line and usually leaves B-cell numbers normal, so it does not fit very low B-cell counts.