Case Presentation: We present a 53-year-old Caucasian male with a history of Ankylosing spondylitis and Sjogren’s syndrome who presented to the ED with cough, fever, fatigue and mild rash that appeared during his admission for a recent knee replacement. Discharged on cefadroxil, his symptoms persisted after completing treatment. No concerns for surgical site infection were found. The respiratory panel confirmed mycoplasma, leading to discharge with doxycycline and antihistamines for urticaria. Later that evening, the patient returned with a worsening diffuse maculopapular rash affecting his back, face, arms, and perineal area associated with few vesicles along with itching and pain, prompting admission due to concerns about Stevens Johnson Syndrome (SJS). While inpatient, dermatology attributed the rash to a cefadroxil-related drug reaction. The patient mentioned prior use of Cephalosporin without any urticaria. Patient was started on high dose steroids and antihistamines for cutaneous eruptions. The following day, skin lesions improved, and the patient was discharged home with the same medications.

Discussion: New-onset cutaneous lesions can stem from infections, medications, food, and insect stings. Rarely, it may signal an underlying systemic disorder, such as urticarial vasculitis, mastocytosis, or systemic lupus erythematosus. Due to rapidly progressing cutaneous lesions and suspicion for developing SJS, the patient was admitted. Cutaneous lesions were attributed to empiric Cefadroxil, and an allergic reaction to cephalosporins was prematurely diagnosed. Doxycycline was started for ongoing mycoplasma, and antihistamines and steroids were administered for cutaneous lesions. In our case, prior awareness of the mucocutaneous-mycoplasma association could have prompted testing during the initial admission, leading to doxycycline use instead of Cefadroxil, preventing readmission and morbidity, and avoiding wrongful implicating an effective antibiotic class.Mucocutaneous involvement with Mycoplasma includes maculopapular or vesicular rash, urticaria, erythema multiforme, and SJS, occurring in about 17% of respiratory tract involvement. Pathophysiology is believed to be from secondary immune phenomena.We recommend that in patients with new cutaneous lesions, and a concurrent or recent URI, a positive PCR test for Mycoplasma can identify a possible association. Prematurely labeling it as a drug or food allergy should be avoided, and biopsy or referral to Allergy and immunology is recommended. Appropriate antibiotic treatment, in such cases, can address refractory mucocutaneous disease. Identifying this association can therefore prevent unnecessary and costly diagnostic workup and prolonged hospital stay.

Conclusions: Commonly, acute urticaria is associated with a presumed drug or food allergy. Maintaining a high-grade suspicion for mycoplasma infection when encountering patients with urticaria is crucial. This enables physicians to inquire about symptoms related to respiratory tract infections and investigate recent exposure. Given the diagnostic challenges, due to subclinical and varied presentations of mycoplasma, this knowledge aids in not only diagnosing and treating mycoplasma early but also addressing associated refractory cutaneous lesions to conventional treatments. Furthermore, this proactive approach helps prevent misclassification of patients as allergic to an entire class of antibiotics, safeguarding their therapeutic options in the future.