Question

A $34$-year-old male presents to the outpatient clinic reporting a painless rash on his hands and feet that appeared $3$ days ago. Over the past $6$ weeks, he has experienced intermittent low-grade fevers, night sweats, generalized joint aches, and a $5$-kg ($11$-lb) weight loss. One month ago, he experienced a mild upper respiratory illness that was treated with azithromycin by a telehealth provider. He has no significant past medical history, though he recalls being told he had a "harmless heart noise" as a child. He takes no daily medications and denies intravenous drug use.

Vital signs include a temperature of $38.2^\circ\text{C}$ ($100.8^\circ\text{F}$), blood pressure of $128/72 \text{ mmHg}$, and heart rate of $94/\text{min}$. Physical examination reveals multiple small, erythematous, non-tender macules on the palmar and plantar surfaces, as well as a few dark red, linear streaks under the fingernails. Cardiopulmonary examination reveals a grade II/VI blowing, decrescendo diastolic murmur heard best at the right second intercostal space. The spleen is palpable $3 \text{ cm}$ below the left costal margin.

Laboratory results are as follows:
Hemoglobin: $10.4 \text{ g/dL}$
Leukocytes: $11,200/\text{mm}^3$
Platelets: $380,000/\text{mm}^3$
Creatinine: $1.5 \text{ mg/dL}$ (Baseline: $0.9 \text{ mg/dL}$)
Rheumatoid factor: $85 \text{ IU/mL}$ (Normal: $<15 \text{ IU/mL}$)

Urinalysis shows $2+$ protein, $2+$ blood, $15$-$20$ RBCs/hpf, and occasional red blood cell casts.

Which of the following is the most likely diagnosis?