**Core Concept**
Priapism is a prolonged and often painful erection of the penis that is unrelated to sexual stimulation. This condition can be caused by various factors, including certain medications, underlying medical conditions, and exposure to toxic substances.

**Why the Correct Answer is Right**
Cantharide poisoning, also known as Spanish fly, is a well-documented cause of priapism. Cantharides, the active ingredient in the Spanish fly beetle, contains a toxin that stimulates the nervous system and causes smooth muscle contraction, leading to an erection. This effect is due to the inhibition of phosphodiesterase and the subsequent increase in cyclic guanosine monophosphate (cGMP) levels, which causes smooth muscle relaxation and blood vessel dilation.

**Why Each Wrong Option is Incorrect**
**Option A:** Rat poisoning is not typically associated with priapism. Rat poisoning usually involves substances like warfarin or bromethalin, which have different mechanisms of action.

**Option B:** Arsenic poisoning can cause a range of systemic symptoms, including gastrointestinal and neurological effects, but it is not typically associated with priapism.

**Option C:** Sildenafil poisoning would more likely cause hypotension and priapism is a known side effect of the drug, but this is not a poisoning scenario. Sildenafil is a prescribed medication for erectile dysfunction.

**Clinical Pearl / High-Yield Fact**
It's essential to note that cantharide poisoning can also cause other symptoms, including gastrointestinal distress, renal failure, and even death, making it a serious condition that requires prompt medical attention.

**Correct Answer:**
✓ Correct Answer: D. Cantharide poisoning.