A 55-year-old woman on corticosteroids for nephrotic syndrome develops a solitary, firm, deep-seated nodule on the thigh that slowly liquefies and fluctuates like a cold abscess, with overlying skin initially normal. Aspiration shows scanty acid-fast bacilli and cultures grow M. tuberculosis. This lesion results from:
- A Contiguous extension from an underlying infected joint
- B Direct inoculation of bacilli through minor trauma in a non-sensitised host
- C Haematogenous dissemination from a distant primary focus in a host with depressed cell-mediated immunity ✓
- D Autoinoculation from sputum onto abraded perioral skin
Correct answer: C. Haematogenous dissemination from a distant primary focus in a host with depressed cell-mediated immunity
Explanation
This is a metastatic tuberculous gumma, also called metastatic cold abscess, arising when bacilli seed the skin haematogenously from a distant focus during periods of impaired immunity such as steroid therapy, malnutrition, malignancy or HIV. The overlying skin appears normal until late fluctuation, distinguishing it from scrofuloderma where skin involvement follows contiguous spread from an infected gland or bone. Inoculation variants require external trauma and a non-immune host.
Reference: Fitzpatrick's Dermatology, 9th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.