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A 9-year-old girl with a history of environmental allergies presents to an outpatient pediatric clinic in the winter with complaints of subjective fever, myalgias, and polyarthralgia. Her symptoms began 5 days before presentation, starting with a limp and generalized lower extremity pain, including bilateral plantar pain. On day 3 of illness, her mother noted a transient erythematous rash on the bilateral forearms that had since resolved and ongoing fingertip pain in both hands with limited manual dexterity. On the day of presentation she reported sore throat.She has not experienced any similar prior episodes and denies weight loss, night sweats, animal exposures, or recent travel. Notably, the day before her presentation, her younger sibling had a brief episode of isolated subjective fever that resolved without further symptoms. There is no family history of autoimmune disease or cancer.On examination, the patient is afebrile with normal vital signs and growth parameters. There is pain with internal rotation of the right shoulder and flexion of the right wrist against resistance. The affected and surrounding joints exhibit a full range of motion without pain on palpation, erythema, warmth, swelling, or effusion. An antalgic gait is evident with apprehensive movement and reluctance to bear full weight on her feet. The physical examination is otherwise notable for palpable bilateral mobile and tender posterior cervical lymph nodes. There are no ophthalmologic, nasal, otic, or oropharyngeal abnormalities. No concerning findings are revealed on skin, chest, and abdominal examinations.Point-of-care group A Streptococcus throat and influenza A/B nasal swab results are negative. A nasal SARS-CoV-2 RNA swab is sent for analysis along with serological tests for Epstein-Barr virus polymerase chain reaction (PCR) tests and parvovirus B19 immunoglobulin (Ig) M/IgG levels. Nonsteroidal anti-inflammatory medication is prescribed for pain. The family is informed of the patient’s possible diagnoses and importance of documenting fever at home with plans for further laboratory investigation if needed.Fever, cervical lymphadenopathy, rash, and migratory limb pain are nonspecific signs of systemic inflammation with a broad differential that spans infectious, autoimmune, and oncologic processes. Given the acute onset, seasonality, and sick contact, infectious causes were considered highest on the differential. Primary considerations included group A Streptococcus (Centor score = 3: <15 years of age, fever, and absence of cough), seasonal influenza, SARS-CoV-2 (based on seasonality and local surveillance data), Epstein-Barr virus (cervical lymphadenopathy, fever, and rash), and parvovirus B19 (polyarthralgia, fever, rash, cervical lymphadenopathy, and pharyngitis).1 Particular attention was paid to parvovirus B19 because it uniquely explains the combination of transient rash, symmetric polyarthralgia, and the absence of high-grade fever or true arthritis.2We considered enteroviral infection—a common summer and early-fall cause of self-limited myalgias and rash—but the winter timing and lack of oral/hand-foot vesicles made this less probable. Acute rheumatic fever was considered given the presence of migratory arthralgia; however, the absence of objective arthritis, carditis, chorea, and prior confirmed streptococcal pharyngitis failed to meet the Jones criteria.Postinfectious and systemic inflammatory conditions were also included in the differential diagnosis. Multisystem inflammatory syndrome in children related to SARS-CoV-2 was lower on the differential given the mild illness and lack of gastrointestinal or cardiovascular involvement. A serum-sickness–like reaction was also entertained given rash and arthralgia, yet the rash was transient. Autoimmune entities such as systemic juvenile idiopathic arthritis (sJIA) can produce fever, rash, and arthralgia. sJIA typically manifests with quotidian fever spikes and persistent arthritis, both absent here. Finally, malignancy—particularly acute leukemia—remained on the differential because arthralgia, bone pain, fever, and lymphadenopathy may herald hematologic cancer; although unlikely, it should not be overlooked if other etiologies of fever and arthralgia are ruled out or cytopenias develop.Laboratory results confirmed the diagnosis of erythema infectiosum (fifth disease) with polyarthralgia syndrome with elevated serum parvovirus B19–specific IgM (17.8 IU/mL; reference <0.9 IU/mL) and IgG (5.1 IU/mL; reference <0.9 IU/mL) antibodies. Epstein-Barr virus whole-blood DNA and rapid Streptococcus A throat swab and culture results were negative along with nucleotide amplification tests for influenza A/B and COVID-19. A complete blood count with differential was not obtained. Although the confirmatory diagnosis of parvovirus B19 was ultimately straightforward, the clinical presentation presented a diagnostic challenge because of the absence of the classic pediatric presentation of slapped-cheek rash (no available photography of rash or rash on physical examination) and symmetric limb and finger pain without overt arthritis combined with cervical lymphadenopathy and rash, which overlaps with the presentation of several inflammatory conditions underscoring the importance of thorough history taking, physical examination assessment, and prioritization of the differential diagnosis.Parvovirus B19 is transmitted through respiratory secretions, percutaneously through contact with blood products, and vertically during pregnancy. The clinical course of erythema infectiosum characteristically involves an initial phase of “flu-like” symptoms followed by rash and/or joint involvement, which coincides with antibody production. Although classically described as “slapped cheek” in appearance, the rash of erythema infectiosum may also present on the extremities as “lace-like” and evanescent, provoked by sun exposure, strong emotions, or warmth.3 Joint involvement is often symmetric, typically involving the hands, wrists, and lower extremities. Arthralgia and rash are secondary to immune-complex deposition and resolve in a few weeks without long-term sequelae.3The diverse manifestations of parvovirus B19 are often associated with host characteristics: erythema infectiosum, typically in young children; petechial, papular-purpuric gloves-and-socks syndrome in children and young adults; and polyarthralgia syndrome, classically in adult women. Parvovirus B19 is a single-stranded DNA virus that replicates in erythrocyte precursors, which may result in apoptosis of these cells and aplastic crisis in susceptible hosts, such as those with hemolytic anemias (eg, sickle cell anemia or autoimmune hemolytic anemia), and can cause hydrops fetalis in the fetuses of infected pregnant women.The peak prevalence for parvovirus B19 infection is in the late winter through early spring. The incubation period ranges from 4 to 21 days; affected individuals are likely no longer infectious at the time of symptom onset. Serum antibodies and PCR for viral load are used for diagnostic evaluation. In immunocompromised individuals, prolonged viral shedding can occur, and IgG/IgM testing may be misleading given compromised Ig production. PCR-based detection is therefore preferred in this case. Droplet precautions are advised in the inpatient setting, and standard hygiene practices (eg, hand washing) are recommended to prevent viral spread.4 At this time, there are no additional precautionary measures recommended for pregnant women given the low incidence of hydrops fetalis relative to the prevalence of prior immunity to parvovirus B19.2Barring severe anemia or aplastic crisis, most immunocompetent hosts recover with supportive measures. Arthralgia can last for several weeks but importantly is not associated with joint destruction.3,4The patient had improvement in arthralgias on telemedicine follow-up 4 days later and reported alleviation of residual mild pain with nonsteroidal anti-inflammatory medications. She had returned to school and resumed her usual activities.The rash of erythema infectiosum is not always a “slapped cheek” in appearance; it may present with an evanescent rash on the extremities.Although classically affecting adults, consider parvovirus B19 polyarthralgia syndrome in children presenting with a limp in the appropriate clinical context.Parvovirus B19–induced arthralgia is caused by immune-complex deposition and is not associated with joint damage.Most patients with parvovirus infection are noninfectious at the time of symptomatic presentation.