**Question:** The decrease in O2 affinity of hemoglobin when the PH of blood falls is called

A. Bohr effect
B. Haldane effect
C. Hofmeister effect
D. Haldane effect (also known as the Hofmeister effect)

**Correct Answer:** D. Haldane effect (also known as the Hofmeister effect)

**Core Concept:**
The Bohr effect and Haldane effect are related phenomena which describe how changes in pH can affect the binding of oxygen to hemoglobin (Hb). These effects are crucial to understand in the context of tissue hypoxia, respiratory alkalosis, and acidosis. The Hofmeister effect is a related concept, also known as the Haldane effect.

**Why the Correct Answer is Right:**
The Haldane effect occurs when the pH of blood decreases, leading to a decrease in the affinity of hemoglobin (Hb) for oxygen. This means that at a lower pH, less oxygen is bound to Hb, which can result in tissue hypoxia (decreased oxygen supply to tissues). The Haldane effect is due to the increased negative charge on the Hb molecule, causing electrostatic repulsion between Hb molecules and reducing oxygen binding.

**Why Each Wrong Option is Incorrect:**
A. Bohr effect: This is the opposite of the Haldane effect. When pH decreases, the affinity of Hb for oxygen increases, causing a higher oxygen saturation in the blood and potentially leading to respiratory alkalosis.

B. Haldane effect (Hofmeister effect): This option is incorrect as the Haldane effect (also known as the Hofmeister effect) refers to the same phenomenon as the correct answer D. The difference lies in the terminology used.

C. Haldane effect: This option is incorrect as it refers to a different concept than the Haldane effect (Hofmeister effect). The Haldane effect describes the decreased oxygen affinity due to low pH, while the Haldane effect refers to increased oxygen affinity when pH decreases.

**Clinical Application:**
Understanding the Haldane effect is essential for diagnosing and treating patients with respiratory acidosis, as it helps clinicians determine the appropriate treatment strategy (e.g., increasing PCO2 or administering oxygen therapy).

**Why Haldane effect is also known as Hofmeister effect:**
The term "Hofmeister effect" was first coined by Austrian biochemist Oswald Avery, who studied the effects of different ions on enzyme activity. The term "Haldane effect" is more commonly used in the context of oxygen binding to hemoglobin. However, the phenomenon described is the same, and the terminology used varies among experts and regions.