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What is Dactylitis in children due to: and explain in detail?
Ref. The Short Textbook of Pediatrics. Pg 644
 

Dactylitis (also called hand-foot syndrome) is painful swelling of the hands or feet.
This is often the first symptom that may show up in small infants or toddlers with sickle cell disease. 
Dactylitis is caused by sickled red blood cells blocking the small blood vessels in the hands and feet.

 
Other causes

TB
Sarcoidosis
Syphilis
Spondyloarthritis
Infection - Commonly caused by skin infections, such as Streptococcus or Staphylococcus