Review Question - QID 212440

A 23-year-old woman comes to the clinic complaining of a rash on her legs for the past week. She reports that 2 weeks ago she had an episode of congestion, runny nose, and cough. Shortly afterwards, she began noticing purplish-red dots appearing on her legs, as shown in Figure A. The patient reports that the rash is not painful, itchy, and is not warm upon palpation. She denies joint pain, weight changes, nausea, diarrhea, or palpitations. Physical examination demonstrates an oral ulcer at the right cheek and flat, red, non-blanching lesions on her legs bilaterally. Laboratory findings are shown below.

Hemoglobin: 13.7 g/dL
Hematocrit: 41 %
Leukocyte count: 7,600/mm^3 with normal differential
Platelet count: 90,000/mm^3

Serum:
Na+: 137 mEq/L
Cl-: 97 mEq/L
K+: 4.6 mEq/L
HCO3-: 25 mEq/L
Glucose: 120 mg/dL
Creatinine: 1.1 mg/dL
Anti-nuclear antibodies: negative

What is the most likely explanation for this patient’s symptoms?