**Core Concept**
Delirium and dementia are both cognitive disorders, but they have distinct underlying pathophysiologies and clinical presentations. Delirium is a state of altered consciousness characterized by acute onset of confusion, disorientation, and fluctuating cognitive impairment, whereas dementia is a chronic, progressive condition marked by gradual decline in cognitive function, particularly memory loss.

**Why the Correct Answer is Right**
Delirium is often accompanied by an altered sensorium, which refers to a disturbance in the level of consciousness, such as reduced awareness of the environment, disorientation, and impaired ability to focus, sustain, or shift attention. This altered sensorium is a key feature that differentiates delirium from dementia, where patients typically remain oriented and aware of their surroundings. The altered sensorium in delirium is thought to result from disturbances in neurotransmitter systems, such as acetylcholine and dopamine, which regulate attention and arousal.

**Why Each Wrong Option is Incorrect**
**Option A:** Loss of memory is a hallmark of dementia, particularly Alzheimer's disease, but it is not a reliable feature to differentiate delirium from dementia, as both conditions can present with memory impairment.
**Option B:** Apraxia, which refers to difficulty with motor tasks, is not a distinctive feature of either delirium or dementia, and its presence does not reliably differentiate between the two conditions.
**Option C:** Delusion, a fixed false belief, is more commonly associated with psychotic disorders, such as schizophrenia, and is not a characteristic that reliably differentiates delirium from dementia.

**Clinical Pearl / High-Yield Fact**
A useful mnemonic to remember the key features of delirium is "CAGE": Confusion, Altered sensorium, Gait disturbance, and Emotional lability. This mnemonic can help clinicians quickly identify patients at risk for delirium and initiate appropriate interventions.

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