What deficiency is known to cause night blindness, skin infections, loss of appetite, and diarrhea?

The deficiency associated with night blindness, skin infections, loss of appetite, and diarrhea is Vitamin A. This vitamin plays a crucial role in maintaining healthy vision, particularly in low-light conditions; a deficiency can lead to impaired vision in dim light, known as night blindness. Additionally, Vitamin A is essential for maintaining the integrity of skin and mucosal surfaces, so a lack of this nutrient can result in an increased susceptibility to skin infections. The vitamin also supports immune function, which is essential for fighting infections, and its deficiency can lead to a compromised immune response, further contributing to susceptibility to illness. Furthermore, a deficiency in Vitamin A can affect appetite and digestive health, resulting in symptoms like loss of appetite and diarrhea. This makes Vitamin A deficiency a key factor in the symptoms listed in the question.