**Core Concept**
Rheumatic fever is an autoimmune inflammatory process that occurs following a group A beta-hemolytic streptococcal infection. It can manifest as a spectrum of clinical syndromes, including carditis, arthritis, chorea, and erythema marginatum.

**Why the Correct Answer is Right**
The manifestation of rheumatic fever that disappears completely is chorea. Chorea is a neurological disorder characterized by involuntary, purposeless movements. In the context of rheumatic fever, chorea is thought to result from an autoimmune response against the basal ganglia, leading to damage and disruption of normal motor function. The movements associated with chorea can be quite disabling, but in most cases, they resolve completely within weeks to months after the onset of symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** Arthritis is a common manifestation of rheumatic fever, but it does not disappear completely. In many cases, the joint inflammation persists and can lead to chronic arthritis.

**Option B:** Erythema marginatum is a rash that can occur in rheumatic fever, but it is not a manifestation that disappears completely. The rash can be intermittent and may not resolve completely.

**Option C:** Carditis is a serious manifestation of rheumatic fever that can lead to significant morbidity and mortality. It does not disappear completely and can result in long-term cardiac damage and dysfunction.

**Clinical Pearl / High-Yield Fact**
Chorea is a classic manifestation of rheumatic fever that resolves completely in most cases. However, it is essential to diagnose and treat the underlying streptococcal infection promptly to prevent the development of more severe manifestations of rheumatic fever.

**Correct Answer:** D. Chorea.