**Core Concept**
A functional disorder is a condition where the body's systems or organs are not functioning normally, but no underlying structural abnormality can be identified. This type of disorder is often characterized by symptoms that are not explained by any identifiable cause, such as a tumor or infection.

**Why the Correct Answer is Right**
Functional disorders are often diagnosed by ruling out other potential causes of symptoms. In the absence of any structural abnormalities, the focus shifts to the body's physiological responses and how they may be contributing to the symptoms. For example, in irritable bowel syndrome (IBS), the bowel is not structurally abnormal, but the muscles in the bowel wall may be overactive, leading to symptoms of abdominal pain and changes in bowel habits.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a functional disorder is not the same as a disorder caused by an underlying structural abnormality, such as a tumor or infection.
**Option B:** This option is incorrect because a functional disorder is not necessarily related to psychological factors, although stress and anxiety can sometimes exacerbate symptoms.
**Option C:** This option is incorrect because a functional disorder is not the same as a metabolic disorder, which is caused by an imbalance in the body's chemical processes.

**Clinical Pearl / High-Yield Fact**
Functional disorders are often challenging to diagnose because they do not have a clear cause, and the symptoms can be nonspecific. However, by focusing on the body's physiological responses and using a combination of medical history, physical examination, and diagnostic tests, clinicians can often identify underlying patterns and develop effective treatment plans.

**Correct Answer: D. Disorder without identifiable cause.**