A 3-year-old boy presented with mild left foot pain 6 weeks ago, which resulted in limping 3 weeks ago, followed by swelling over the dorsum of the foot less than a week ago. He was previously healthy except for perianal abscess which developed 1 week after birth.He had met (and exceeded several of) his development milestones, and was up to date with all local vaccines.

An MRI of his left foot was done.

talus
MRI of the young boy’s left foot, showing increased signal intensity around his talus bone.

An orthopaedic surgeon performed a bone biopsy, with necrotising granulomatous inflammation seen on histology, but no fungi or acid-fast bacilli were seen.

Question: What is the likely diagnosis here? And how should he be managed?

[Updated 11 April 2016]

This is likely mycobacterial infection given the absence of other risk factors, despite the rarity of such cases. Besides TB, one other serious consideration is BCG infection, given the age of the child. Although it is not a compulsory vaccination (only diphtheria and measles vaccines are mandatory in Singapore), most newborns in Singapore still undergo the BCG vaccination. There was a marked increase in BCG-related infections (mostly lymphadenitis) in Singapore in 2011, for which no good reason was provided by the HSA report. This was also the year that this particular boy was vaccinated.

Bone cultures did turn out to be positive for Mycobacterium bovis (BCG strain), for which he was treated accordingly. Testing for Mendelian susceptibility to mycobacterial disease (MSMD) was also offered.

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