Cassandra, a middle-aged grade school teacher, presents in the office with a 1-week history of rash on her right lower leg. The eruption is painless, but she's worried as it doesn't seem to go away on its own and that it may be connected to something she touched in the classroom. She's been healthy otherwise, except for mild hypertension that's controlled with beta-blockers.

On exam, there are several discrete erythematosus spots on her leg, ranging from medium to dark red, and measuring about 1 cm in diameter. They're neither nodular nor vesicular, and not tender nor painful. For the most part, they're flat, macular and seem to originate from under the skin surface.

Further physical examination reveals a new heart murmur. It's a soft systolic 2/6 flow type murmur that's heard loudest at the 3^rd intercostal space on her left side. The first heart sound (S1), indicating the closing of the tricuspid valve that marks the beginning of systole, is normal. Diastolic heart sound S2 seems fine as well, though it's split in timing, as the pulmonary valve shuts a little later than the aortic. There's no S3, which is a good sign in a person of that age, as it would indicate a ventricular vibration at diastole, consistent with aortic or mitral valve regurgitation or heart damage. S4, an atrial vibration correlating with the second phase of ventricular filling when the atria contract, shouldn't be heard, and in fact, I don't find it.

The rest of the physical exam reveals a woman looking her stated age. She's a bit heavy and her body mass index is about 32 kg/m². Her blood pressure measures 130/ 70 mm Hg. There's no difference between the right and left arm and no evidence of reduced blood flow to her upper and lower extremities.

So what's causing Casandra's rash?