### **Official Solution & Concept Analysis (GATE BT 2023 Q.26)**
**1. Governing Concept**:
- **Topic**: Bioinformatics & Sequence Similarity Searching
- **PSI-BLAST** (Position-Specific Iterative BLAST) builds a position-specific scoring matrix (PSSM) from an initial set of aligned hits and iteratively searches the database. This profile-based search is substantially more sensitive than standard BLAST and is specifically designed to detect **remote/distant protein homologs** sharing low sequence identity (
lt; 25\%$, twilight zone).
**2. Option Analysis**:
- **Option (A) [BLASTN] is INCORRECT**: Compares nucleotide query against nucleotide database.
- **Option (B) [BLASTX] is INCORRECT**: Compares 6-frame translated nucleotide query against protein database.
- **Option (C) [PSI-BLAST] is CORRECT**: The premier bioinformatic tool for discovering remote, distant protein homologs.
- **Option (D) [TBLASTX] is INCORRECT**: Compares 6-frame translated nucleotide query against 6-frame translated nucleotide database.
**3. Final Answer**:
- **Correct Choice**: **C**