# A Wolf in Sheep's Clothing: Extensive Musculoskeletal and Cutaneous TB Masquerading as Primary Erythema Nodosum

## TL;DR

A man with a history of TB and a weakened immune system was misdiagnosed with a skin condition for months before being correctly diagnosed with musculoskeletal tuberculosis.

## Contribution

Highlights the diagnostic challenges of musculoskeletal TB and the importance of considering TB in immunocompromised patients with atypical symptoms.

## Key findings

- The patient's symptoms were initially misdiagnosed as erythema nodosum and bacterial cellulitis.
- MRI and intraoperative cultures confirmed musculoskeletal TB with pan-sensitive M. tuberculosis.
- TB should be considered in immunocompromised patients with symptoms unresponsive to standard treatments.

## Abstract

Tuberculosis (TB) causes substantial morbidity and mortality globally, with continued endemicity in developing populations. Most cases of primary TB present as pulmonary TB; however, between 10% and 20% include extrapulmonary manifestations. Almost 26% of extrapulmonary manifestations include musculoskeletal TB. The risk of reactivation of latent TB is approximately 10% per year and is most common in those with immunocompromising conditions. Diagnosis of extrapulmonary TB remains elusive because of atypical presentation.

We present a case of a 71-year-old man with a history of renal transplantation and previously treated latent TB who presented with worsening left lower extremity pain and swelling over the previous six months. Initially, the patient was unsuccessfully treated for presumed bacterial cellulitis with several rounds of antibiotics. The patient was then seen by Dermatology, who diagnosed presumed primary erythema nodosum. He was managed with prednisone and topical steroids for several months with only partial improvement. At admission, physical exam was significant for a 4 × 3-cm erythematous, warm, and tender area on the lower left extremity. Labs showed elevated neutrophils and lymphopenia, and an MRI of the limb suggested hindfoot abscesses, talonavicular septic arthritis, and surrounding osteomyelitis. Intraoperative cultures grew acid-fast bacilli and were confirmed to be pan-sensitive M. tuberculosis complex.

Musculoskeletal TB diagnosis requires a high index of suspicion, thorough physical exam, imaging, and tissue for culture and biopsy. Special attention should be placed on the combined risk factors of previous TB diagnosis, immunocompromised status, and symptoms refractory to alternative management strategies such as antibiotics and corticosteroids.