Bone-related disorders: Elevated alkaline phosphatase (alk phos) with normal liver enzymes (AST, ALT, bilirubin) often points towards bone-related conditions. The most common of these is osteomalacia or rickets in children, where there's softening of the bones due to deficiency of vitamin D, calcium, or phosphate. In adults, osteoporosis or Paget's disease of bone could also be considered, though these might not always present with isolated alk phos elevation.

Hepatobiliary diseases with a cholestatic pattern: Although liver enzymes like AST and ALT are normal, and bilirubin is within the normal range, conditions that cause cholestasis (bile flow obstruction) can lead to elevated alk phos. This includes primary biliary cirrhosis, primary sclerosing cholangitis, and choledocholithiasis (gallstones in the bile ducts).
Vitamin D deficiency: This can lead to an increase in alk phos due to its effects on bone metabolism, independent of liver function.

Malignancy: Certain cancers, especially those involving the bone (like metastatic bone disease from prostate, breast, or lung cancer) or the liver/bile ducts (like cholangiocarcinoma), can present with elevated alk phos. Missing these diagnoses could have severe consequences.
Budd-Chiari syndrome: A condition characterized by thrombosis of the hepatic veins, which can lead to liver dysfunction and elevated alk phos, even if other liver enzymes are initially normal.

Hypophosphatasia: A rare genetic disorder characterized by deficient activity of tissue-nonspecific alkaline phosphatase, leading to elevated levels of certain substrates and low alk phos activity in some forms, but can present with elevated alk phos in others.
Wilson's disease: Although typically associated with abnormalities in AST, ALT, and bilirubin due to liver involvement, some cases might present with isolated alk phos elevation, especially in the early stages.