A 4-year-old child presents with a painless, slowly enlarging submandibular lymph node over 6 weeks. The node is firm, mobile, and not accompanied by fever, weight loss, or a positive tuberculin skin test. Aspirate culture yields scotochromogenic colonies that produce yellow pigment even when incubated in the dark. The most likely agent is:

• A Mycobacterium kansasii
• B Mycobacterium tuberculosis
• C Mycobacterium scrofulaceum ✓
• D Mycobacterium avium complex

Explanation

M. scrofulaceum is the classic cause of paediatric cervicofacial lymphadenitis among nontuberculous mycobacteria. It is a scotochromogen in Runyon group II, forming orange-yellow pigment in both light and darkness, which distinguishes it from photochromogens such as M. kansasii that pigment only after light exposure. Children typically have no constitutional symptoms and may have a negative tuberculin test because these species share few cross-reactive antigens with M. tuberculosis.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.