Downward displacement of the cerebellar vermis, fourth ventricle, and brainstem through the foramen magnum is a hallmark finding of Arnold-Chiari II malformation. This hindbrain herniation produces the characteristic banana sign on fetal ultrasound, due to an abnormally curved cerebellum, and the lemon sign, which reflects frontal skull contour changes. These features often occur with spinal neural tube defects such as myelomeningocele, making Chiari II a classic association. Other conditions have distinct imaging patterns. Dandy-Walker malformation shows an enlarged posterior fossa with cystic dilation of the fourth ventricle and vermian abnormalities, not downward herniation through the foramen magnum. An arachnoid cyst is a CSF-filled space that doesn’t produce the hindbrain herniation pattern typical of Chiari II. Hydranencephaly involves near-complete loss of cerebral hemispheres and lacks this hindbrain displacement scenario.

The endometrium has a superficial functional layer that responds to the monthly hormonal cycle and a deeper basal layer that remains after shedding. During the cycle, the functional layer thickens and becomes highly vascularized in preparation for possible implantation. If pregnancy does not occur, withdrawal of progesterone causes the functional layer to shed, resulting in menses. The basal layer remains intact and serves as the source to regenerate a new functional layer in the next cycle. The myometrium, the muscular layer surrounding the endometrium, is not shed. So the layer that is shed during menses is the functional layer.

The blood from the gallbladder, pancreas, spleen, and gastrointestinal tract is collected by the portal venous system and delivered to the liver. The main portal vein is the central trunk that receives tributaries from these organs: the splenic vein brings blood from the spleen (and parts of the pancreas), the superior mesenteric vein drains much of the GI tract (and also receives pancreatic venous blood), and cystic veins drain the gallbladder into the portal system. All these streams converge into the main portal vein, which then carries nutrient-rich venous blood to the liver for processing. After passing through the liver, blood exits via the hepatic veins into the inferior vena cava. The other vessels don’t serve as the direct collection point for blood from all these organs: the inferior vena cava handles systemic return, the hepatic artery supplies arterial blood to the liver, and the superior mesenteric vein is a major tributary feeding the portal system but does not alone receive blood from the gallbladder and spleen.

A focal outpouching of one arterial wall is a saccular aneurysm. This shape is a true aneurysm where a localized segment of the vessel wall balloons out on one side, creating an opening or neck. It contrasts with a fusiform dilation, which enlarges the entire circumference of the vessel in a spindle-shaped way. The description also helps distinguish from a pseudoaneurysm, which is not a true aneurysm: it results from a breach in the vessel wall with blood contained by surrounding tissue, forming a hematoma that communicates with the artery rather than an outpouching of the wall itself. Mycotic aneurysm refers to an infection-driven true aneurysm and can be saccular in shape, but the hallmark descriptor here is the focal, pouch-like bulge of a single wall.

Obturator internus forms the lateral margins of the true pelvis around the obturator foramen. It begins on the inner surface of the obturator membrane and the surrounding pelvic bones, then passes through the lesser sciatic foramen to reach its insertion on the greater trochanter of the femur. This placement makes it the muscular boundary on the lateral wall where the obturator foramen sits. The piriformis is located more posteriorly as part of the posterior pelvic wall, the iliopsoas runs along the anterior aspect near the pelvic brim, and levator ani makes up the pelvic floor rather than the lateral wall.