**Core Concept**
Renal tubular disease is characterized by impaired function of the renal tubules, which can lead to electrolyte imbalances, acid-base disturbances, and abnormalities in urine composition. The renal tubules play a crucial role in reabsorbing essential nutrients and ions, regulating electrolyte balance, and maintaining acid-base homeostasis.

**Why the Correct Answer is Right**
The correct answer is indicative of renal tubular disease because it reflects a disturbance in the normal reabsorption and secretion processes of the renal tubules. This can lead to an abnormal level of a particular substance in the blood or urine. For example, a low level of certain ions (e.g., potassium, magnesium) or an abnormal level of certain substances (e.g., glucose, amino acids) in the urine can indicate impaired renal tubular function. The correct answer is often a specific ion or substance that is normally reabsorbed or secreted by the renal tubules.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specifically indicate renal tubular disease. While it may be related to kidney function, it is not a direct indicator of renal tubular dysfunction.
**Option B:** This option is incorrect because it is a normal finding in many individuals and does not specifically indicate renal tubular disease.
**Option C:** This option is incorrect because it is more indicative of renal glomerular disease, which affects the filtration function of the kidneys rather than the reabsorption and secretion processes of the renal tubules.

**Clinical Pearl / High-Yield Fact**
In cases of renal tubular disease, it is essential to carefully evaluate the patient's electrolyte and acid-base status, as well as their urine composition, to guide appropriate management.

**Correct Answer:** D.