Keyhole sign is seen in
**Core Concept**
The keyhole sign is an important radiological finding seen on imaging studies, particularly in the context of abdominal or pelvic pathology. It refers to a characteristic appearance on imaging where a small, rounded structure is surrounded by a larger, more prominent structure, resembling a keyhole.
**Why the Correct Answer is Right**
The keyhole sign is typically associated with a dilated ureter surrounding a constricted or kinked portion, often due to external compression. This compression can be caused by various factors such as a tumor, fibrosis, or a kink in the ureter. The dilated ureter above the constriction and the constricted ureter below it create the characteristic keyhole appearance on imaging studies like CT or MRI.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the keyhole sign is not typically associated with gallbladder pathology. While gallbladder disease can cause various imaging findings, the keyhole sign is more specific to ureteral pathology.
**Option B:** This option is incorrect because the "bull's eye" sign is a different radiological finding, typically seen in the context of liver or pancreatic pathology, where a central scar is surrounded by a ring of hypoattenuation.
**Option C:** This option is incorrect because the "target sign" is another radiological finding, often seen in the context of liver or pancreatic pathology, where a central scar or nodule is surrounded by a ring of enhancement.
**Option D:** This option is incorrect because the "halo sign" is a different radiological finding, often seen in the context of lung or pleural pathology, where a central area of consolidation is surrounded by a ring of ground-glass attenuation.
**Clinical Pearl / High-Yield Fact**
The keyhole sign is an important radiological finding that can be seen in various contexts, but it is most commonly associated with ureteral pathology. Recognizing this sign can help clinicians diagnose and manage conditions such as ureteral obstruction or tumor.
**Correct Answer: D.