**Core Concept**
Type I diabetes mellitus (DM) is an autoimmune condition characterized by the destruction of pancreatic beta cells, leading to a deficiency in insulin production. This results in hyperglycemia due to impaired glucose uptake by cells.

**Why the Correct Answer is Right**
Option B, "Antibodies against β cells," is the correct answer because it accurately describes the pathogenesis of type I DM. In this condition, the immune system produces autoantibodies against the islet cells of the pancreas, specifically targeting the β cells responsible for insulin production. This process results in the destruction of β cells and a subsequent deficiency in insulin production. The presence of these autoantibodies is a hallmark of type I DM and is often used as a diagnostic marker.

**Why Each Wrong Option is Incorrect**
**Option A:** "90% family H/O" is incorrect because while a family history of type I DM may be present in some cases, it is not a universal characteristic of the condition. Only about 15% of individuals with type I DM have a first-degree relative with the disease.

**Option C:** "Insulin given to Rx DKA" is incorrect because while insulin is a crucial treatment for diabetic ketoacidosis (DKA), type I DM is characterized by a deficiency in insulin production, not an excess. Insulin is given to treat DKA, but it is not a primary treatment for type I DM.

**Option D:** "D.K.A occurrence" is incorrect because DKA is a complication of type I DM, but it is not a defining characteristic of the condition. DKA occurs when there is a severe deficiency in insulin production, leading to hyperglycemia and metabolic acidosis.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize that type I DM is an autoimmune condition characterized by the presence of autoantibodies against β cells. This understanding is crucial for accurate diagnosis and management of the condition.

**Correct Answer: B. Antibodies against β cells.**