Substance Intoxication or Withdrawal: This is often the most common cause of altered mental status in young adults, given the prevalence of substance use. Alcohol, opioids, cocaine, and other substances can cause a range of alterations in mental status, from mild confusion to coma.

Infection (e.g., Meningitis, Encephalitis): Infections affecting the central nervous system can present with altered mental status, fever, headache, and sometimes focal neurological signs.
Traumatic Brain Injury: Given the age group, trauma is a significant consideration. Altered mental status could be due to a concussion, subdural hematoma, or other types of intracranial hemorrhage.
Metabolic Disorders (e.g., Diabetic Ketoacidosis, Hypoglycemia): These conditions can cause altered mental status due to the direct effects of metabolic derangements on the brain.
Psychiatric Conditions (e.g., Psychotic Break, Severe Depression): While psychiatric conditions might not always cause an acute alteration in mental status, they can certainly lead to changes in behavior and cognitive function that might be perceived as altered mental status.

Status Epilepticus: A life-threatening condition that requires immediate recognition and treatment. It can present with altered mental status, especially if the patient is in a post-ictal state.
Cerebrovascular Accident (Stroke): Although less common in younger adults, stroke can occur and is a medical emergency. Altered mental status can be a presenting symptom, especially in cases of posterior circulation strokes.
Septic Shock: Sepsis can cause altered mental status due to the systemic inflammatory response and potential multi-organ failure, including cerebral dysfunction.
Carbon Monoxide Poisoning: This can cause altered mental status due to hypoxic brain injury and is crucial to diagnose early, as treatment (hyperbaric oxygen) is time-sensitive.

Autoimmune Encephalitis: A group of conditions characterized by an immune-mediated attack on the brain, leading to altered mental status, seizures, and sometimes focal neurological signs.
Prion Diseases (e.g., Creutzfeldt-Jakob Disease): Extremely rare but fatal neurodegenerative disorders that can present with rapidly progressive dementia and altered mental status.
Mitochondrial Disorders: A group of genetic disorders affecting the mitochondria, which can lead to a variety of neurological symptoms, including altered mental status, especially during periods of high energy demand or metabolic stress.
Wilson's Disease: A genetic disorder leading to copper accumulation in the brain and liver, which can cause neurological symptoms, including altered mental status, particularly in young adults.