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What is Mongolian spot and explain in detail?
A Mongolian spot, also known as slate grey nevus or congenital dermal melanocytosis, is a benign, flat, congenital birthmark with wavy borders and an irregular shape. In 1883, it was described and named after Mongolians by Erwin Bälz, a German anthropologist based in Japan, who erroneously believed it to be most prevalent among his Mongolian patients. It normally disappears three to five years after birth and almost always by puberty. The most common color is blue, although they can be blue-gray, blue-black or deep brown.

Cause
Mongolian spot is a congenital developmental condition—that is, one existing from birth—exclusively involving the skin. The blue colour is caused by melanocytes, melanin-containing cells, that are usually located in the surface of the skin (the epidermis), but are in the deeper region (the dermis) in the location of the spot. Usually, as multiple spots or one large patch, it covers one or more of the lumbosacral area (lower back), the buttocks, sides, and shoulders. It results from the entrapment of melanocytes in the lower half to two-thirds of the dermis during their migration from the neural crest to the epidermis during embryonic development.Male and female infants are equally predisposed to slate grey nevus. People who are not aware of the background of the slate grey nevus may mistake them for bruises, possibly resulting in mistaken concerns about abuse.

Anthropological description
The French anthropologist Robert Gessain interested himself in what he called the tache pigmentaire congenitale or coloured birthmark, publishing multiple papers in the Journal de la Société des Américanistes, an academic journal covering the cultural anthropology of the Americas. Gessain spent time with the Huehuetla Tepehua people in Hidalgo, Mexico, and wrote in 1947 about the spots "location, shape, colour, histology, chemistry, genetic transmission, and racial distribution". He had previously spent several winters in Greenland, and wrote an overview in 1953 of what was known about the spot. He hypothesised that the age at which it faded in various populations might prove to be a distinguishing characteristic of those groups. Gessain claimed that the spot was first observed amongst the Inuit.Hans Egede Saabye, a Danish priest and botanist, spent 1770–1778 in Greenland. His diaries, published in 1816 and translated into several European languages, contained much ethnographic information. He described the spot on newborns, saying he had seen it often when the infants were presented naked for baptism. A second Danish observer was doctor and zoologist Daniel Frederik Eschricht, mainly based in Copenhagen. In 1849 he wrote of the "mixed" babies he had delivered at the lying-in hospital. He also says that "the observation made for the first time by Saabye about Inuit children has been completely confirmed by Captain Holbøll", who sent him a fetus pickled in alcohol.Gessain goes on to state that it was only in 1883 that an anthropologist mentions the spot. It was Erwin Bälz, a German working in Tokyo, who described a dark blue mark on Japanese infants. He presented his findings in 1901 in Berlin, and from that point on, Bälzs name was associated with certain skin cells containing pigment. Captain Gustav Frederik Holm wrote in 1887 that his Greenlandic interpreter Johannes Hansen (known as Hanserak) attested to the existence of the birthmark over the kidney region of newborns, which grows larger as they grow older. That year, the Danish anthropologist Soren Hansen drew the connection between the observations of Bälz in Japan and Saabye in Greenland. "This cannot be a coincidence. It is not the first time that the resemblance between the Japanese and the Eskimo has been pointed out." Fridtjof Nansen, the Norwegian polar explorer, said that the spot was widespread in the mixed Danish-Inuit population of West Greenland. Soren Hansen confirmed this. A missionary in Bethel, Alaska, a traditional gathering place of Yupik people, reported that the spots were common on children. Rudolf Trebitsch, an Austrian linguist and ethnologist, spent the summer of 1906 on the West Coast of Greenland, and listed all the examples he came across. Gessain went to north Labrador in 1926, looking for children with these spots. In 1953 Dr Saxtorph, medical advisor to the Greenland department (part of the Danish government), wrote that the Greenlanders do not like outsiders to see or discuss these birthmarks; "they doubtless feel as a reminiscence of the time when they lived on a low cultural level".The presence or absence of the slate grey nevus was used by racial theorists such as Joseph Deniker (1852-1918), the French anthropologist.The Journal of Cutaneous Diseases Including Syphilis, Volume 23 contained several accounts of the slate grey nevus on children in the Americas:

Holm ("Ethnological Sketch. Communications on Greenland," X., Copenhagen, 1887) announced the presence of the spot in the east part of Greenland. Bartels ("The So-Called Mongolian Spots on Infants of Esquimaux," Ethnologic Review, 1903) received letters regarding it from East Greenland and also from Esquimaux of Alaska. In half-breed European-Esquimaux, Hansen says he has encountered it. Among Indians of North Vancouver, British Columbia, there are observations made by Baelz as well as by Tenkate (secondhand). In the Mayas of Central America, Starrs (Data on the Ethnography of Western Mexico, Part H., 1902) facts are corroborated by Herman (Aparecimiento de la Mancha Mongolica. Revista de Ethnologia, 1904). He cites A. F. Chamberlain (Pigmentary Spots, American Anthropologist, 1902,) and Starr (Sacral Spots of Mayan Indians, Science, New Series, xvii., 1903).
In Central America, according to these authorities, the spot is called Uits, "pan," and it is an insult to speak of it. It disappears in the tenth month. It is bluish-reddish (in these Native people), and is remarkable by its small size. The mulberry colored spot is very well known in Afro-Brazilians. In Brazil, among individuals of mixed Indigenous American and West African descent (pardo) it is called "genipapo", from its resemblance in color (bluish-gray) to an indigenous fruit of Brazil, named genipapo (a Native word adopted into Portuguese).

Prevalence
Infants may be born with one or more slate grey nevus ranging from small area on the buttocks to a larger area on the back. The birthmark is prevalent among East, South, Southeast, North and Central Asian peoples, Indigenous Oceanians (chiefly Micronesians and Polynesians), certain populations in Africa, Amerindians, non-European Latin Americans and Caribbeans of mixed-race descent.They occur in around 80% of Asians, and 80% to 85% of Native American infants. Approximately 90% of Polynesians and Micronesians are born with slate grey nevus, as are about 46% of children in Latin America, where they are associated with non-European descent. These spots also appear on 5–10% of babies of full Caucasian descent; Coria del Río in Spain has a high incidence due to the presence of descendants of members of the delegation led by Hasekura Tsunenaga, the first Japanese official envoy to Spain in the early 17th century. African American babies have slate grey nevus at a frequencies of 90% to 96%.According to a 2006 study examining the Mongolian spot among newborns in the Turkish city of İzmir, it was found out that 26% of the examined babies had the condition. It was noted the prevalence rate was 20% and 31% in boys and girls, respectively