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What is Urticaria pigmentosa and explain in detail?
Urticaria pigmentosa (also known as generalized eruption of cutaneous mastocytosis (childhood type): 616 
) is the most common form of cutaneous mastocytosis.  It is a rare disease caused by excessive numbers of mast cells in the skin that produce hives or lesions on the skin when irritated.

Signs and symptoms
Urticaria pigmentosa is characterized by excessive amounts of mast cells in the skin.  Red or brown spots are often seen on the skin, typically around the chest, forehead, and back.  These mast cells, when irritated (e.g. by rubbing the skin, heat exposure), produce too much histamine, triggering an allergic reaction that leads to hives localized to the area of irritation, sometimes referred to as Dariers sign.  Severe itching usually follows, and scratching the area only serves to further symptoms.  Symptoms can be mild (flushing and hives that require no treatment), moderate (diarrhea, tachycardia, nausea/vomiting, headache, and fainting), or life-threatening (vascular collapse requiring emergency treatment and hospitalization).

Cause
The majority of urticaria pigmentosa cases are caused by a point mutation at amino acid 816 of the proto-oncogene c-kit.  c-kit is a transmembrane protein which, when bound to Mast Cell Growth Factor (MCGF), signals the cell to divide.  Mutations in position 816 of c-kit can result in a constant division signal being sent to the mast cells, resulting in abnormal proliferation.  Different mutations have been linked to different onset times of the disease.  
For example, the Asp816Phe and Asp816Val mutations (the aspartate normally at position 816 in the c-kit protein has been replaced with phenylalanine or valine respectively) have been associated with early manifestation of the disease (mean age of onset: 1.3 and 5.9 months respectively). 
The c-kit gene is encoded on the q12 locus of chromosome 4.

Irritants
Several factors can worsen the symptoms of urticaria pigmentosa:
Emotional stress
Physical stimuli such as heat, friction, and excessive exercise
Bacterial toxins
Venom
Eye drops containing dextran
NSAIDs
Alcohol
MorphineThe classification of NSAIDs can be disputed.  Aspirin, for example, causes the mast cells to degranulate, releasing histamines and causing symptoms to flare.  However, daily intake of 81 mg aspirin may keep the mast cells degranulated. Thus, while symptoms may be worsened at first, they can get better as the mast cells are unable to recharge with histamine.

Diagnosis
The disease is most often diagnosed as an infant, when parents take their baby in for what appears to be bug bites.  The bug bites are actually the clumps of mast cells.  Doctors can confirm the presence of mast cells by rubbing the babys skin.  If hives appear, it most likely signifies the presence of urticaria pigmentosa.

Treatments
There are no permanent cures for urticaria pigmentosa