## Core Concept
Delirium is a neurocognitive disorder characterized by acute and fluctuating disturbances in attention, awareness, and memory. The condition often results from medical conditions, substance intoxication or withdrawal, or medication side effects. Memory in delirium can be affected, but certain types are more preserved than others.

## Why the Correct Answer is Right
Procedural memory, also known as implicit memory, involves the recall of skills and habits, such as riding a bicycle or playing a musical instrument. This type of memory is relatively preserved in individuals with delirium. Delirium primarily affects short-term memory (episodic memory) and the ability to form new memories (anterograde amnesia), which are aspects of explicit memory. The pathophysiology of delirium involves disruptions in neurotransmitter systems and brain regions critical for attention and short-term memory, such as the prefrontal cortex and the parietal lobe, but tends to spare procedural memory.

## Why Each Wrong Option is Incorrect
**Option A:** Short-term memory is significantly impaired in delirium. This type of memory involves holding and manipulating information over short periods, which is characteristically disturbed in delirium.

**Option B:** Long-term memory can be variably affected but is not as consistently spared as procedural memory. Episodic long-term memory (recalling specific events) may be impaired, especially for events occurring during the episode of delirium.

**Option C:** Working memory, a component of short-term memory, is also impaired in delirium. It involves the ability to hold information in mind and manipulate it over short periods, crucial for complex tasks.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that patients with delirium may have difficulty recalling recent events or learning new information, but they may still be able to perform tasks they learned in the past, such as riding a bike. This distinction can be helpful in assessing and managing patients with delirium.

## Correct Answer: D. Procedural memory.