Answer:

Introduction:
Glucose is a simple sugar that is normally filtered by the kidneys and reabsorbed back into the bloodstream. However, in certain medical conditions, glucose is lost in the urine. One such condition is diabetes mellitus. Let's discuss why glucose is lost in the urine of patients suffering from diabetes mellitus.

Explanation:
Diabetes mellitus is a metabolic disorder characterized by high blood sugar levels due to either insufficient insulin production or the body's inability to use insulin effectively. In this condition, glucose cannot enter the cells and remains in the bloodstream, leading to hyperglycemia.

Role of Kidneys:
The kidneys play a crucial role in maintaining glucose homeostasis. They filter the blood, reabsorb essential substances, and excrete waste products in the form of urine. Normally, almost all of the filtered glucose is reabsorbed back into the bloodstream by the renal tubules.

Effect of Diabetes Mellitus:
In diabetes mellitus, the high levels of glucose in the blood exceed the renal threshold for glucose reabsorption. The renal threshold is the blood glucose concentration at which glucose starts to be excreted in the urine. In healthy individuals, the renal threshold for glucose is approximately 180-200 mg/dL.

Key Point 1: Renal Threshold for Glucose:
In diabetes mellitus, the blood glucose levels are consistently high, exceeding the renal threshold. As a result, glucose is not entirely reabsorbed by the renal tubules, and some amount of glucose is lost in the urine.

Key Point 2: Polyuria:
The presence of excess glucose in the urine leads to an osmotic diuresis, causing increased urine production (polyuria). This occurs because glucose in the urine attracts water molecules, resulting in increased fluid loss.

Key Point 3: Glycosuria:
The presence of glucose in the urine is called glycosuria. It is a hallmark feature of diabetes mellitus and can be detected through a urine test.

Conclusion:
In summary, glucose will be lost in the urine of patients suffering from diabetes mellitus due to the high blood glucose levels exceeding the renal threshold for glucose reabsorption. This condition leads to polyuria and glycosuria, which are characteristic features of diabetes mellitus.