A 40-yeаr оld femаle presents in the emergency rооm with diаrrhea, abdominal cramps and gas. She has no other physical symptoms and no allergies. She told the PA student taking her medical history that she had just left a birthday party at which she consumed significant amounts of ice cream. When asked if she usually eats dairy foods, she responds that she cut dairy from her diet several years ago because eating it made her stomach hurt. Her blood tests reveal low levels of which digestive enzyme and the diagnosis of which of the following disorders.