In primary hyperparathyroidism, the parathyroid gland secretes too much PTH regardless of calcium levels. This leads to elevated serum calcium, which should normally suppress PTH production.

The pathergy test involves pricking the skin with a sterile needle. If a small bump or pustule forms within 48 hours, it indicates skin hyperreactivity, which is characteristic of Behçet's disease.

A thyroid scan helps determine the function of a nodule. 'Cold' nodules do not take up iodine and have a higher risk of being malignant, whereas 'hot' nodules are usually hyperfunctioning and benign.

The pituitary gland is extremely sensitive to small changes in thyroid hormone levels. Therefore, an elevated TSH is the most sensitive and earliest indicator of primary hypothyroidism.

A non-contrast CT scan is the first-line and highly sensitive test for acute subarachnoid hemorrhage, appearing as hyperdense (white) areas in the subarachnoid spaces.

Intussusception occurs when one part of the intestine slides into another. The 'currant jelly' appearance of the stool is caused by a mixture of mucus and blood from the compromised intestinal lining.

Pernicious anemia is an autoimmune condition where the body attacks parietal cells or intrinsic factor. Testing for anti-intrinsic factor antibodies is highly specific for this diagnosis.

Colonoscopy is considered the gold standard for both detection and prevention (via polyp removal). Stool-based tests like FIT or gFOBT are also effective screening options.

Standard X-rays are often negative for the first 2–3 weeks of a stress fracture. A bone scan or MRI can detect the increased metabolic activity or bone marrow edema much earlier.

The aPTT measures the integrity of the intrinsic and common pathways. It is used to monitor heparin therapy and screen for deficiencies in factors VIII, IX, XI, and XII.