**Core Concept:** Erectile dysfunction (ED) is a common urological complaint characterized by persistent inability to achieve or maintain an erection sufficient for satisfactory sexual intercourse. It may be caused by organic factors like vascular, neurological, hormonal, or psychogenic factors. In this case, basic screening evaluation is unremarkable, indicating the need to explore other pathophysiological mechanisms.

**Why the Correct Answer is Right:** In this scenario, the correct answer is **D** (perform a penile Doppler ultrasound). A penile Doppler ultrasound is a non-invasive diagnostic test that evaluates blood flow to the penile tissues. It is useful in identifying organic causes of erectile dysfunction, such as vascular abnormalities or erectile tissue damage. This helps to differentiate psychogenic from organic causes of ED and guides appropriate treatment strategies.

**Why Each Wrong Option is Incorrect:**

A. **Echocardiogram**: This test evaluates heart function and is not relevant to evaluating erectile dysfunction.

B. **Semen analysis**: This test evaluates semen quality and is not directly related to assessing the cause of erectile dysfunction.

C. **Hormonal profile**: Although hormonal abnormalities can contribute to ED, specifically addressing the hormonal profile without considering vascular or neurological factors is not comprehensive enough.

**Clinical Pearl:** In cases of unexplained ED after basic evaluation, a penile Doppler ultrasound is the next logical step to identify organic causes and guide treatment strategies. A thorough assessment involving the evaluation of vascular, neurological, hormonal, and psychogenic factors is essential in determining the most appropriate treatment plan for the patient.