Question:

This is a seventy seven year old man who has been complaining of a persistent of erythematous plaque on his right anterior shin for more than fifteen years. He has enjoyed good past health and he is not concerned about the rash except that it is a bit unsightly. The rash is not painful or itchy. It has been there for many years with gradual increase in size. On examination, there were similar rashes found on the back and buttock. He occasionally found the rash less sensitive to change of temperature and sharp points. He was a farmer in a village of Guangdong before he came to Hong Kong forty years ago.

Answer: D. Leprosy

Leprosy is a relatively rare chronic skin infection in Hong Kong nowadays. There were only seven new leprosy infections notified in Hong Kong in year 2003. Most of these cases contracted the infection decades ago and none of them were born in Hong Kong. Only two of the seven new cases were born in the Mainland and the rest were born in South-east Asia. Leprosy or Hansen's disease is a chronic granulomatous disease caused by Mycobacterium leprae principally damaging peripheral nerves and skin. Depending on the host immune response, clinically leprosy can be classified into Tuberculoid, Borderline and Lepromatous leprosy. Skin smear, skin or nerve biopsy are sometimes needed to confirm the infection. Those with a high host immunity tend to develop a paucibacillary tuberculoid type of leprosy in which no leprosy bacilli will be identified from skin smear but skin biopsy will confirm the histopathology findings of typical granuloma around the nerves. The destruction of peripheral nerves can be confirmed by careful testing of sensory nerves over the affected area and in patients with any skin rash with diminished pin prick, temperature and touch sensation, leprosy must be excluded. This patient is suffering from borderline leprosy with annular granulomatous erythematous plaque persistent in the same location for many years. Psoriasis will not be static and stable in the same site for decades. Mycosis fungoides should not have any alteration in cutaneous sensation but it can also persist for a long time before typical features of tumour stage are found. Squamous cell carcinoma usually will not involve multiple sites and is too slow to progress if it is a malignant skin condition. Skin biopsy is essential to confirm the true nature of a persistent granulomatous skin rash.