**Core Concept**
NSAID-induced allergic interstitial nephritis is a form of acute kidney injury caused by nonsteroidal anti-inflammatory drug (NSAID) hypersensitivity. This condition is characterized by an immune-mediated response to NSAIDs, leading to inflammation and damage in the kidneys' interstitial tissue.

**Why the Correct Answer is Right**
Heavy proteinuria, which is the presence of an abnormal amount of protein in the urine, is a common feature of NSAID-induced allergic interstitial nephritis. This occurs due to the damage to the glomerular basement membrane and the loss of its selective permeability, allowing proteins to leak into the urine. The immune response and subsequent inflammation in the kidneys lead to this disruption in the glomerular filtration barrier.

**Why Each Wrong Option is Incorrect**
**Option A:** Fever is a non-specific symptom that can be associated with various conditions, including infections and inflammatory disorders. While fever may be present in some cases of NSAID-induced allergic interstitial nephritis, it is not a specific or common feature of this condition.

**Option B:** Rash is a symptom more commonly associated with allergic reactions, such as urticaria or angioedema, rather than interstitial nephritis. Although a rash may occur in some cases of NSAID hypersensitivity, it is not a defining feature of NSAID-induced allergic interstitial nephritis.

**Option C:** Eosinophilia, which is an increase in eosinophil count in the blood, is not typically a feature of NSAID-induced allergic interstitial nephritis. Eosinophilia is more commonly seen in conditions such as parasitic infections, asthma, or certain types of vasculitis.

**Clinical Pearl / High-Yield Fact**
Be aware that NSAID-induced allergic interstitial nephritis can present with non-specific symptoms, making it challenging to diagnose. Heavy proteinuria is a key feature that should raise suspicion for this condition, especially in patients taking NSAIDs and presenting with acute kidney injury.

✓ Correct Answer: D. Heavy proteinuria