With reference to the negative feedback system shown in the figure above, which of the following descriptions best describes the receptors (marked A in the figure) used to detect decreases in blood calcium concentrations? 
Definitions:
Intrusive Recollections
Unwanted and repeated memories of traumatic events that vividly come back to a person's mind, causing distress.
Social Withdrawal
The behavior of avoiding social interactions and situations, often due to anxiety, depression, or other factors.
PTSD
Post-Traumatic Stress Disorder, a mental health condition triggered by experiencing or witnessing a terrifying event, characterized by flashbacks, nightmares, severe anxiety, and uncontrollable thoughts about the event.
Obsessive-Compulsive Disorder
A mental disorder characterized by unwanted, recurring thoughts and repetitive behaviors that a person feels compelled to perform.
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