Susan is 28. Two weeks ago she was sick, then noticed severe thirst despite drinking plenty. She also lost a few pounds from her usual slender frame. Lab work at the doctor's included a blood sugar >400 and ketones in her urine. Susan most likely has diabetes.

Susan most likely has Diabetes Mellitus

Explanation: Diabetes Mellitus is an endocrine disorder/metabolic disorder that is characterised by severe deficiency, absence or resistance of the hormone insulin due to the destruction of cells of the pancreas (beta cells).

The primary sign of Diabetes Mellitus is hyperglycemia which is an increase in the blood glucose levels followed by a relative disruption of carbohydrate and fat metabolism in the body.

Diabetes Mellitus raises the renal(kidney) threshold for glucose, the glucose remaining in the filtrate raises it's osmotic pressure,water reabsorption is reduced and the volume of urine is increased.

Due to relative insensitivity of the cells of the body to insulin,the cells are essentially starved of glucose because in the absence of insulin, they are unable to use the glucose from the bloodstream leading to muscle wasting, tissue breakdown and and increase in blood glucose level.

In the absence of insulin to promote normal glucose metabolism, alternative energy sources are used instead by the body and increased breakdown of fat occurs.This brings about the production of acidic ketone bodies which can be used for metabolism by the liver.

When ketones accumulates in the blood,it is excreted via the urine.(Ketonuria)