Psychogenic ED: This is often the most common cause of ED in young men, related to performance anxiety, stress, or relationship issues. The age of the patient and the lack of other specified symptoms make this a likely first consideration.

Lifestyle Factors: Smoking, alcohol use, drug abuse, and lack of physical activity can contribute to ED. Given the age, these factors are common and could easily contribute to ED.
Depression and Anxiety: These mental health conditions are prevalent in young adults and can cause or exacerbate ED.
Hypogonadism: Low testosterone levels can lead to ED, and while less common, it's a possibility, especially if there are other signs of hypogonadism.

Testicular Cancer: Though rare, testicular cancer can lead to hormonal imbalances that might cause ED. It's crucial to examine for testicular masses.
Peyronie's Disease: This condition, characterized by fibrous scar tissue inside the penis, can cause painful erections and ED. Early diagnosis is important for treatment.
Diabetes Mellitus: Early onset diabetes can lead to neuropathy and vascular disease, contributing to ED. It's essential to screen for diabetes, especially if there are other risk factors.

Pituitary Tumors: Tumors affecting the pituitary gland can disrupt hormone production, including testosterone, leading to ED.
Neurological Conditions: Conditions like multiple sclerosis, Parkinson's disease, and spinal cord injuries can cause ED, though they are less likely in a 20-year-old without other neurological symptoms.
Vascular Diseases: While rare in young adults, conditions like atherosclerosis or vascular malformations could potentially cause ED.
Medication Side Effects: Certain medications, including some antidepressants, antihistamines, and blood pressure medications, can cause ED as a side effect. This would be more relevant if the patient is taking any prescription medications.