**Core Concept**
Sodium loss, also known as hyponatremia, occurs when the concentration of sodium ions in the blood falls below 135 mmol/L. It can lead to various clinical manifestations due to the shift of water from the extracellular to the intracellular space, causing cerebral edema and altered mental status.

**Why the Correct Answer is Right**
The earliest clinical indicator of sodium loss is altered sensorium, which refers to a change in the level of consciousness or mental status. This is because the brain is highly sensitive to changes in sodium levels, and even mild hyponatremia can cause cerebral edema, leading to symptoms such as confusion, disorientation, and altered sensorium. The mechanism behind this is the movement of water into brain cells due to the osmotic gradient, causing swelling and increased intracranial pressure.

**Why Each Wrong Option is Incorrect**
**Option B:** Reduced skin turgor is a sign of dehydration, which is a consequence of sodium loss, but not the earliest clinical indicator.
**Option C:** Arrhythmia can occur in severe cases of sodium loss, but it is not the earliest clinical indicator.
**Option D:** Orthostatic hypertension is not a typical finding in sodium loss; in fact, orthostatic hypotension is more common due to the decreased effective circulating volume.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the earliest clinical indicator of sodium loss is altered sensorium, and prompt recognition and treatment are crucial to prevent further neurological complications.

**✓ Correct Answer: A. Altered sensorium**