Answer to Mid-Month Mindbender February 2013

Rajalakshmi T, Inchara YK

Pityriasis rubra pilaris

The “checkerboard” sign consists of alternating, short ortho- and parakeratosis in both vertical and horizontal directions within the stratum corneum (see image below.) Also note that the normal basket-weave cornification is retained above this zone and the granular layer is normal or slightly thickened beneath it.  
Although specific, this sign is often subtle and is difficult to find. In early stages of the disease, pityriasis rubra pilaris shows small mounds of parakeratosis, similar to psoriasis. But, the two can be distinguished by the presence of accompanying neutrophils in the parakeratotic foci as well as diminished granular layer in psoriasis.