A 4-year-old child has slowly enlarging unilateral cervical lymph nodes over several months. The nodes are nontender, and the child is afebrile with no pulmonary findings. Excision biopsy shows caseating granulomas; culture yields colonies that turn yellow-orange even when grown away from light. Which organism is most likely?

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

Explanation

Mycobacterium scrofulaceum is the classic Runyon group II scotochromogen: colonies are pigmented (yellow-orange) both in the dark and after light exposure. It is a recognised cause of isolated cervicofacial lymphadenitis in young children, treated by surgical excision. M. tuberculosis causes tuberculous lymphadenitis but is nonchromogenic; M. avium complex is also nonpigmented; M. fortuitum is a rapidly growing nonchromogenic saprophyte associated with injection-site and wound infections.

Reference: Ananthanarayan and Paniker Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.