**Core Concept**
Erythrocyte Sedimentation Rate (E.S.R.) is a blood test that measures how quickly erythrocytes (red blood cells) settle at the bottom of a test tube containing a blood sample. This rate is influenced by various factors, including the concentration and size of red blood cells, as well as the presence of acute phase reactants such as fibrinogen and immunoglobulins.

**Why the Correct Answer is Right**
Polycythemia rubra vera is a myeloproliferative disorder characterized by an overproduction of erythrocytes, leukocytes, and platelets. In contrast to other conditions listed, polycythemia rubra vera typically results in a decreased E.S.R. due to the increased viscosity of the blood, which impairs the settling of red blood cells. This is because the increased number of red blood cells makes it more difficult for them to move and settle at the bottom of the test tube.

**Why Each Wrong Option is Incorrect**
**Option A:** Polymyositis rheumatica is a type of inflammatory arthritis that often presents with elevated E.S.R. levels due to the production of acute phase reactants such as fibrinogen and C-reactive protein.

**Option B:** Multiple myeloma is a plasma cell dyscrasia characterized by the overproduction of monoclonal immunoglobulins, which can lead to an elevated E.S.R. due to the increased concentration of these proteins in the blood.

**Option C:** Temporal arteritis is a form of large vessel vasculitis that often presents with elevated E.S.R. levels, particularly in the absence of other obvious causes of inflammation.

**Clinical Pearl / High-Yield Fact**
When interpreting E.S.R. results, it is essential to consider the clinical context and potential causes of abnormal values. In the case of polycythemia rubra vera, a decreased E.S.R. may be seen due to the increased viscosity of the blood, highlighting the importance of considering the underlying diagnosis when interpreting laboratory results.

**✓ Correct Answer: D. Polycythemia rubra.**