**Core Concept**
The diagnosis of major depressive disorder (MDD) is based on the presence of specific symptoms that meet the criteria outlined in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5). One of the key symptoms required for a diagnosis of MDD is a persistent and significant loss of interest or pleasure in activities that were once enjoyable.

**Why the Correct Answer is Right**
Loss of interest or pleasure, also known as anhedonia, is a critical symptom of MDD. It is characterized by a significant reduction in the ability to experience pleasure from activities that were once enjoyable, such as hobbies, social interactions, or even sex. This symptom is thought to be related to alterations in the brain's reward system, particularly the dopamine and serotonin pathways. The presence of anhedonia is a key differentiator between MDD and other mood disorders, such as dysthymia or adjustment disorder.

**Why Each Wrong Option is Incorrect**
**Option B:** Recurrent suicidal tendency is a symptom that can be associated with MDD, but it is not a required criterion for diagnosis. While suicidal ideation or behavior is a serious concern in patients with MDD, it is not a substitute for the core symptoms of the disorder.

**Option C:** Insomnia is a common symptom of MDD, but it is not a required criterion for diagnosis. Insomnia can be a presenting symptom of MDD, but it can also be a symptom of other sleep disorders or medical conditions.

**Option D:** Indecisiveness is a symptom that can be associated with MDD, but it is not a required criterion for diagnosis. Indecisiveness can be a manifestation of the cognitive symptoms of MDD, such as difficulty concentrating or making decisions.

**Clinical Pearl / High-Yield Fact**
Remember the mnemonic "DEPRES" to help you recall the core symptoms of MDD: **D**epression, **E**nergy loss, **P**ersistent anhedonia, **R**ecurrent thoughts of death, **E**xcessive guilt, and **S**leep disturbances. This mnemonic can help you remember the key symptoms of MDD and distinguish it from other mood disorders.

**✓ Correct Answer: A. Loss of interest or pleasure**