MKSAP Quiz: 2-day history of painful rash


A 22-year-old man is evaluated in the emergency department for a 2-day history of painful rash on the left side of his posterior chest. Medical history is unremarkable, and he takes no medications.

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On physical examination, temperature is 37.5°C (99.5°F), blood pressure is 115/62 mm Hg, pulse rate is 78/min, and respiration rate is 20/min. A vesicular rash is shown.

Which of the following is the most appropriate test to perform next?

A. CH50 activity

B. Fourth generation HIV-1/2 antigen/antibody combination immunoassay

C. IgA measurement

D. Quantitative immunoglobulin measurement


Answer and critique

The correct answer is B. Fourth generation HIV-1/2 antigen/antibody combination immunoassay. This item is Question 85 in MKSAP 18's Infectious Disease section.

The most appropriate test to perform next is a fourth generation HIV-1/2 antigen/antibody combination immunoassay. This test combines an immunoassay for HIV antibody with a test for HIV p24 antigen. This improves the ability of the test to detect early HIV infection because p24 antigen becomes detectable a week before antibody in acute infection. Detection of antigen may help diagnose patients as early as 2 weeks after infection. This patient's painful vesicular rash distributed over a thoracic dermatome is classic for infection with varicella-zoster virus. Older adults and immunocompromised patients, including patients with HIV infection, are at increased risk. Severe or recurrent varicella-zoster virus infections or infection at a young age should prompt an evaluation for HIV infection.

Patients with terminal complement component deficiencies usually present with recurrent, invasive infections with encapsulated bacteria such as Neisseria meningitides, Haemophilus influenzae, and Streptococcus pneumoniae. These patients should be screened for complement deficiency by assaying for CH50 activity. If CH50 activity is normal, alternate pathway function should be assessed with an alternative complement pathway (AH50) assay. If results of either assay are abnormal, specific component concentrations should be determined.

Selective IgA deficiency is one of the most common B-cell immunodeficiencies. Inheritance may be autosomal dominant or recessive; most cases are sporadic. Patients with selective IgA deficiency may be asymptomatic or present with recurrent sinopulmonary infections (otitis media, sinusitis, pneumonia) or gastrointestinal infections (giardiasis). Other common manifestations include inflammatory bowel disease; celiac disease; an increased frequency of autoimmune disorders, including rheumatoid arthritis, systemic lupus erythematosus, and chronic active hepatitis; and allergic disorders, including asthma, allergic rhinitis, and food allergies.

Common variable immunodeficiency involves B- and T-cell abnormalities and results in clinically significant immune dysregulation. The primary manifestation is hypogammaglobulinemia, and recurrent respiratory infections are a common presentation in adults. The gastrointestinal tract is frequently involved and causes malabsorption or chronic diarrhea. Infection with Giardia, Campylobacter, or Yersinia species may occur, as may opportunistic infections. Concurrent autoimmune disorders occur in up to 25% of patients. The risk for malignancy is increased, including gastrointestinal cancers and non-Hodgkin lymphoma. Patients also have a poor or an absent response to protein and polysaccharide vaccines. Serum immunoglobulin levels are usually low, circulating B cells may be normal or low, and T-cell function varies. The diagnosis is made by confirming low levels of total IgG and IgA or IgM, as well as by a poor antibody response to vaccines.

Key Point

  • Infection with varicella-zoster virus in a young patient should prompt testing for HIV infection.